Well the Normans in retrospect, at least in popular culture and probably I think for very good reason, have a particularly ruthless reputation. It would be terrible to be an Anglo-Saxon of some rank under the Norman yoke, and I was mainly thinking of such people being the ones who either stand and with Harald's help hold somewhere in the middle, or succumbing to overwhelming Norman power farther south, flee (or die). The peasants of course are wanted to produce on site, without them the land is worthless--but that hardly led to anything approaching civil rights for them generally in the Middle Ages and it is here especially the Normans have their reputation for putting up with no nonsense and instilling obedience via gross terror. So, such people are not perhaps as able to up and run as their former AS overlords might be, as these fleeing former aristocrats would have money and it would be the habit of the people they move among as refugees to defer to them, and they'd be likely to be able to get some kind of half-respectable position in the north, especially as fighting men. The commoners have nothing and would be viewed with suspicion as a rabble of strangers with no ties and nothing to lose. And of course the Normans are not going to stand around idly while their peasants go wandering off, and will no doubt intercept parties of them and either ascertain where they came from and send them back there, or just bind them to service wherever is convenient to themselves near at hand to where they were caught.
If enough commoners managed to escape the Norman ruled zone, there would be a shortage of them where the rest remained and that might give them some leverage in negotiations.
But again, William and gang came over in force, prepared to fight and defeat Harold's army, and if they arrive at Hastings with no such army to oppose them, I doubt they'd be very conciliatory--on the contrary, the English they face in the south would be largely depleted of fighting men of any great worth, due to such having been conscripted to Harold's banner and then marched off north. No doubt there would be most of the administrative infrastructure such as it was (which I gather by early medieval standards, was quite above average in late Saxon England actually, but as noted, not the necessary men at arms in adequate numbers).
William would have some ATL assets he did not have OTL, offsetting the awkward fact that the Normans have yet to face and, they hope, defeat England's army in detail to firmly establish there is no other effective force remaining in England, which is a definite downside to them at this early point. But on the other hand, William is able, for what it is worth, to take the high road at least briefly and with whatever sincerity or lack of it he displays, and point out he did not kill Harold. With Harold dead at someone else's hands, William might consider assembling the remaining kingdom AS hierarchy, minus so many of their soldiers, and producing the Papal decree proclaiming himself the rightful successor to Edward the Confessor the Witan should have chosen instead of Harold--now Harold is dead, the judgement of God is plain, William was always and definitely is now the only rightful King of England, and if the English will snap to and start obeying him properly the kingdom can go on as before, but better now that God is well satisfied right is being done.
It is the Saxon gentry, not the commoners, I think he'd be addressing and perhaps keeping faith with, if they do come over to him in apparent good faith. And I suppose many might, especially if he keeps his side of the deal.
But will he? Can he even? He has assembled a big force of very haughty and powerful lords, and he has promised them that their reward for prevailing and taking England would be a great increase in their wealth with new manors they would presume they are free to run just as ruthlessly as they were accustomed to in Normandy (and Flanders, I gather a fairly large minority of the "Normans" included Flemish allies)--or perhaps even much more so, taking over with a clean slate of zero obligations to these alien subjects. However desirable it is to get the willing allegiance of the English, he has on hand a force that can bypass that and just impose obedience in a manner they are well accustomed to--and any falling between stools might leave him doubly weakened, with the English dissembling in bad faith and looking for every opportunity to overthrow him, while any restraint he imposes by decree on the Norman barons will be resented by them--they might individually intrigue with the English in various schemes, or collectively set up one of their own to usurp William's claim, or just plain fail to prevail against the English if they are deprived of their usual accustomed methods of rule.
I would hardly put attempting to be all things to all men at least in cheap words past William at this juncture, but he had better be able to signal to the Norman barons in his gang that this is a ruse, and their "rights" as overlords will always take precedence, and that will mean that William will be roundly hated as both cruel and faithless. The more he temporizes with experiments in incorporating English methods and English people, the longer these have to use their traditional channels of communication to come to whatever consensus seems most reasonable to them--and OTL we know, they did not accept the Normans' early terms and so the King and his barons wound up systematically crushing the lot of them.
I have another point I am not so comfortable on--it is all very well for Harald's Norwegian invaders to prevail at Stamford in the sense of managing to kill off Harold himself, that seems entirely possible. But consider that OTL, with Harald dead and Harold alive, the native English king was able to not only kill Harald himself but subdue the invaders well enough that either they got back into their boats and left, or were suitably secured some other way (perhaps some individuals gave paroles and kept them, or even signed on to serve Harold?) so that Harold could then take that army of his, somewhat attritted with battle casualties, and march them fast down to Hastings on very short notice, leaving the North to fend for itself, which I gather happened without any remnant of Harald and Tostig's forces rallying and making any sort of trouble behind Harold's back.
Clearly Harold had a pretty good and pretty large army then, they could not only hold their own but effectively completely crush Harald's lot, and then Harold could reckon he stood some kind of chance against a second invading army that would have a considerable amount of time to dig in in the south. As things were OTL he did not succeed of course--but if the odds had seemed hopelessly against him in Hastings, would his conscripted army, already having served longer than legal terms permitted, have followed him into battle? Would not some delegation of the Witan (yes, I know the Witangemot was not a fixed parliament, it was just the custom of gathering up major bigwigs who more or less spoke for their various regional constituencies--all the more reason then I can suppose some bunch of AS factionalists can just whip one up and figure if they prevail the king will affirm it was a legal thing retroactively) come to Harold and talk turkey with him, pointing out that they could withdraw and attempt a long term war of attrition, or they could negotiate terms, but they cannot just go into this fight and expect to win?
Apparently they thought they could win, and I daresay that outcome, even a second time with an exhausted and somewhat demoralized army, was in the cards.
Personal disclosure--I have a romantic attachment to Harold Godwinson, I sentimentally feel he had bad luck and a raw deal at Hastings, and this was bad for the native English, very bad. I much prefer TLs where he wins in the south too and the Normans have the stuffing kicked out of them, with William deservedly dead and the Norman, and to some degree Flemish, systems back home decapitated and depleted and vulnerable to their Continental foes including of course the French king--I even like to entertain the speculative possibility Harold of Wessex gets together a counter-invasion fleet and perhaps joins forces with the hegemon of Brittany to go grab some Norman territory and perhaps parts of Flanders too. This last bit is hyperbolic and improbable, I suppose honestly.
But this idea of a split England is appealing enough to me to entertain. Having warned I might be blinded by excessive sentiment, I do think soberly though that the hardest part is Harald retaining enough fighting force loyal firstly to him to even survive in the North, never mind take over the crown. It seems plain that even if Harald's forces largely hold and in some military sense can be said to have prevailed in the battle that kills Harold, still the English forces are unlikely to be totally decimated or deranged--the short term chain of command seems unlikely to be broken to me, among the English.
But I am hardly expert in medieval warfare and one thing I do know about it, from taking classes on the Crusades and other general reading, is that chaos played a huge role. It might not be good betting odds, but I suppose it is entirely possible that the northern battle is something of a push, with neither the Norwegian nor the English either massively attritted nor enjoying clear decisive victory. With the news of Harold's death, the immediate response in such a case might be both forces withdrawing to recuperate, and pondering their next move.
Then at this awkward juncture, the news arrives in the north that William has landed.
OTL with Harald dead and the Nordic force apparently whittled down to something local forces could handle reasonably well, Harold immediately marched south. But here, any military successor in command of the English would have to realize marching away without some kind of settlement with the Norse would be a pretty terrible thing for the loyal English of the north, and would buy Harald time to consolidate at least a foothold of control there and leave them a foe to return to fight in a third battle, with depleted English forces.
Whereas if they just stay put and renew the fight again they will be depleted even if victorious, while meanwhile William is consolidating his control in the south unopposed!
Upthread I offered the scenario of a gradual shift of English allegiance toward Harald over time, as the English piecemeal have time to form their own judgements one by one of their odds under either pretender and wherever they can get away with it somehow, conclude they had better help the Viking.
One reason I didn't offer the scenario of a big part of England's established elites collectively and immediately declaring for Harald was that it was hard for me to see how Harald would come out of the initial northern fight with enough force to be a viable contender anymore, and it does seem to me that is the way to bet, but it goes against the OP, so I tried to skate on past it in silence.
Having acknowledged this elephant in the room, the only way I see to play by OP guidelines to suppose that an unlikely but possible outcome where Harald is still standing pretty strong and so is the English army--and the notables more or less collectively in charge of this organized English force do the math pretty quick and conclude their best chance is to accept Harald on the spot, negotiate terms with him, and then join forces to after some brief delay versus OTL, march down south to get rid of the Normans and set Harald up as their new king but with business in England going on much as usual.
Now if fate did play this unlikely hand, I do think that in these circumstances, the English, despite delaying perhaps some weeks or even a month or so before marching down, would have a likely shot at victory. After all, if they don't drop everything and start quick-marching south, when they do arrive somewhat later than OTL, they won't be so exhausted. They might also have time and opportunity to replenish their ranks both by scouring the undisturbed as yet towns and villages for more fighters, and taking charge of any able-bodied male refugees of any social rank who might be fleeing the Normans. Against this of course the Normans have more time to spread out and dig in, and they just might also be able to come to acceptable terms with the English within their reach.
Anyway once again, it would go against the OP for the north English/"Viking" alliance to prevail like that. The idea is to achieve a stalemate of some kind, both in the immediate short run of 1066 and the handful of years after that, and ideally somehow for centuries after. Clearly then it is time to put our thumbs on the scales of probability again and just limit ourselves to gaming out just how it could work out that the northern expedition does not manage to dislodge the Normans, nor do the Normans manage to crush the northern forces completely, and eventually a draw emerges where Harald and successors hold north England and William and gang hold the south.
I guess if we are vague and not too picky about considering only the likeliest things but consider longer shot possibilities, we can have this readily enough actually.
As noted the northern army comes late, but in better order, to the south. Meanwhile William has time, and avoids most of the attrition Hastings might have cost him OTL. For all of what I despise about William, he was definitely a systematic and astute thinker, and so I suspect he'd control the Norman forces tightly and expand out from Kent very systematically. He'd want to advance to the Thames pretty quickly I'd think and get control of London and then westward--and if he refrains from surging northward to thin his forces out too much, deploying a fairly thin but solid buffer across the middle of the south toward Wales would carve off the richest parts of England, and with them the various sites of ceremonial significance--Canterbury is already in hand pretty quickly, and London, and I don't know exactly which other sites are most important to get either culturally or strategically, but while just the southern tier might not look like much on a map, it is the major part demographically and economically I believe. Moving the bulk of his loyal Normans a short distance north means he has to use more carrots than sticks to get tribute and other forms of useful aid from the English trapped to the south, but he might well be able to do so, at least in the short run.
So when the northern army marches south, they find the Normans have advanced considerably in their direction, and have spread out somewhat, and might even be able to recruit some English auxiliaries to more or less reinforce the invading knights.
Also William has some time to send for more forces from the Continent, though I would not put too much weight on that in the short run--for one thing, OTL and presumably here, the landing at Hastings happened at the very last minute one could expect decent sailing weather across the Channel, so even mere communications might be haphazard, and moving big forces might be clean out until the next spring. For another William bet the farm on this Conquest scheme of his and surely took with him in the first wave everyone that he judged Normandy could spare; bringing more over, even some 6-8 months hence, might leave his home duchy in some jeopardy. Probably the way to bet is that he has to pretty much work with whatever Normans survive the upcoming trials by battle and other attritions (plague, Saxon uprisings, etc) that might be in store over the next few years--give it some years, and some profits being expatriated back to Normandy, and he can probably call on minor drafts of reinforcements perhaps as early as late 1067 and reliably a few more every year after that. But he has limited fixed and more declining than rising stocks of reliably loyal Normans to work with. The only way he can expand his forces beyond that is to persuade English men to fight for him too, and I have given my reasons to think that inevitably, even if William manages to be more conciliatory than OTL, he won't have the sentiment of native English strongly on his side--generally.
I do think perhaps, if William can keep his barons in line (and he was pretty good at that, being the son of Robert the Devil and all that, he was something of a Khruschev to Robert's Stalin, or Peter to Robert's Ivan the Terrible I guess, he could play both carrot and stick adroitly, with everyone knowing just how brutal a beating the sticks would give them if it comes to that and so motivated to take whatever carrots they can get) he can keep faith with enough selected English gentry, provided they keep faith with him, to favor this English minority and recruit them into the ranks of Norman barons. The barons themselves would resent this of course and I don't think William can go so far as to tip the balance of his rule in south England against Norman supremacy, entirely, but if suitably loyal and useful English lords can swallow their ethnic resentments and get with the program of Normanizing individually, I suppose some grudging support among some of the Normans for individual guys who are proven loyalists and learning to be suitably ruthless against their own countrymen in Norman fashion would carry over the first generation, whereas the sons (and daughters) of such turncoat Saxo-Norman lords would be quite intensively Normanized--it would be within William's general character and track record I believe for him to establish the condition that the heirs, and backup second sons, and most desirably marriageable daughters of Saxo-Norman trustee lords would customarily ship off to Normandy to be guests at some suitably loyal and connected Norman baron's demesne in Normandy itself--there to be raised exactly as natural born Norman, to learn the language while still young children so as to be fully proficient in it, to build up kinship ties in the same fashion as Norman gentry, and to be intermarried with proper Norman brides, or husbands--either way, the children of a mixed marriage of that type would be deemed and treated as purely Norman, and when the first generation of such coopted Saxon barons on probation die off, it would take a look at tables of genealogy or of course diagnosis of their family surname to determine whose ancestors were English, and whose were Norman, in 1065. In fact the third and fourth generation might be quite thoroughly mixed, with very few able to claim pure Norman ancestry (though fewer could claim pure English either).
On such terms, William and his barons would have to wait longer for the really rich tributes to roll in and they might not ever quite reach OTL levels. Insofar as ongoing warfare with the northern forces keep wearing his limited stock of true blue Norman barons, it is easier to compromise with the English collaborators, but vice versa he'd worry if the ratio of his forces gets to be too English--in later generations this would not be a problem but it is now.
But right now, in late '66 going into '67, they have a somewhat ramshackle and divided northern army to contend with. That army might initially believe that they had best catch all the Normans in one place and destroy them, then hope to move in on south England and restore the old regime under Harald pretty much. But the Normans would spread out a bit I think, maybe proceeding westward more slowly than I imagined and perhaps holding off on invading Anglia for a while, so they still have some decent concentration but in several knots. So the northern forces find they have to either divide forces themselves, or concentrate on just one of these knots heavily, hope to destroy it, then move on to mop up the next and the next. If their first battle is a victory of the type they hope for I would think the Norman incursion is done for--even if William can hold on somewhere, he might find he can only do so with a lot of native English help, the English would be more restive and less loyal, and his Norman barons still standing would be pretty disgruntled with this costly fiasco to boot.
So let's assume the opposite--the Northern forces swoop down on one of the Norman concentrations, figuring their superior numbers make victory a pretty sure thing--and then for the first time in this ATL, the English and Norwegian forces have to face just what advantages Norman continental style knighthood bring them on the battlefield, and find themselves being cut to ribbons. Also they'd be just loosely integrated at this point, and have picked up a lot of green troops who have not even had the experience of Stamford under their belts, and the upshot is they get decimated pretty badly. But with a king on their side, and fairly competent English commanders in parallel with reasonably smart Norwegian commanders, they see what is happening and retreat back north to regroup some distance back. And they start to learn what it would take to hold against Norman knights, a process that might take some years before they figure out how.
To what degree does the North have no choice but to gradually foster such knights themselves, and to what degree can the North English (I think I will look ahead and guess they might wind up centuries hence calling themselves "Northumbrian," though I am hardly married to this and it is anyway anachronistic at this point, but perhaps in retrospect ATL histories will be calling them that so let me for convenience, subject to counterarguments of course!) actually manage survival then the occasional victory with few to no such knight but with asymmetrical warfare innovations of their own, such as anticipating the development of pike formations, or fostering the kind of longbow archery that served the OTL English so well in the Hundred Years War some centuries hence? Did the longbows already have some primordial form in Saxon England somewhere, and if so, were there any good candidates for local culture of it in the northlands?
Related to questions about whether the Northumbrians need knights at all, and if so how many of them, are ones about whether such chivalry can only be raised and supported by wholesale adoption of Norman style brutally hierarchal manorialism, with a large number of deeply exploited and brutalized peasants going to support quite extravagant costs of devoting big and expensive horses trained exclusively for combat ridden by professional knights who also have to drill with little time for other pursuits, and therefore organize all society around their quarrelsome games of rivalry by private combat as the necessary condition for their being available to their overlords and ultimately king in concentration from time to time? Even supposing Northumbria must ultimately raise up and support considerable numbers of men skilled at this type of fighting, are there alternative ways the gradually consolidating north kingdom can sustain and command them on a more modern-looking (or we might as well say, Roman or Eastern Empire style bureaucratized professionalism) drawing collective revenues from taxes rather than direct manorial tributes, and raising these forces to serve in an integrated national army under direct control of the King? Must this class of men wind up also being petty absolute monarchs of their own private domains answering only to very codified feudal norms of legalistic obedience on specified conditions to a feudal network? Or granting a centralized army might exist, must what quasi-republican parliamentarianism Northumbria might evolve be almost entirely a toy of this narrow class of knightly soldiers, with the state riding roughshod collectively in the name of the King on the peasants just as firmly as the Norman barons did piecemeal--or is it possible that a more dispersed and gradual social hierarchy allowing better off burghers and the more prosperous peasants to have their somewhat humbled but still effective say at the high levels of governance and be trusted to a consensual oversight of their own home governance within their communities? Certainly if the knightly forces require a substantial amount of infantry support, as well as perhaps crossbow or even longbowmen, and maybe pike formations, that gives the commoners some leverage perhaps--it hardly did so in formal or apparent terms in many Early Modern societies of course, though perhaps we'd be wrong to underestimate the indirect influences mass peasant/urban artisan interests had on the practice of astute absolutist governance even in places like France in the 18th century or Russia in the next one, or say the Austro-Hungarian empire or Prussia.
Being the sentimentalist I am, I am going to guess the Northumbrians find that with some suitable tactical and strategic innovations of their own they do still require some knight-equivalent heavy cavalry to prevail in fights against the south kingdom, but fewer than the Normans normally depend on, and that they can indeed sidestep the institutionalization of manorial demesnes in favor of general taxation and funding of these specialists by the royal purse in a centralized manner. No doubt social hierarchy remains a rather severe thing overall, with common carls having rather few rights and living hard lives working quite hard, paying high taxes, and still at risk of a lot of social breakdowns in famines and plagues and so on, but the class structure would I would guess remain relatively fluid, it being relatively easy for particular individuals to rise through the ranks, by meritorious and useful service or by accumulating wealth commercially, and climb the social ladder without too much prejudice about humble origins a generation or three back. I think this is pretty much continuous with how society tended to actually work in later Saxon England.
For what it is worth, I do know of one model for medieval chivalry to come under central command and be supported by central royal funding--the Kingdom of Jerusalem. Under Baldwin of Flanders, the KoJ concentrated its knights in urban dwellings, where they were maintained by the King collecting taxes, building on the old Muslim institutions the prior regimes used to tax the Arabic and other ethnic peasants known as "fellahin"--one help here was taking over large land grants that had been donated to fund Muslim charities, such as supporting pilgrims to Palestine and on their way to the Hejaz, or supporting Islamic schools; the "Franks" had no regard for these causes and freely usurped the lands, but put all the revenues into one royal pot, which were then largely used to support the knights in the cities rather than granting the lands for them to disperse into privately run castles as in Europe. I gather this system prevailed as long as the kingdom lasted, and worked well enough for the successive kings. Of course these urbanized knights still had the class attitudes that their more rurally dispersed models in Europe had--in terms of background, quite a few of them were jumped up peasants actually, but they learned knightly fighting on the job of the First Crusade, and were favored because they were after all Latin Christians who spoke the languages of their betters, their intermediate roles being taken over by native Palestinian non-Catholic Christians known as "turcopols."
In the evolving northern English kingdom, I can at least hope that the new knights the kingdom gradually acquires are socially more continuous with the general population, and have no preconceived ideas of their personal entitlement to rule private domains, though no doubt they would come to feel entitled to be supported in lavish style by the realm collectively and probably be pretty assertive politically, as a bloc and as intriguing factionalists to boot.
Meanwhile the model for how we can expect Norman ruled south England to work is plain before us, just look up OTL patterns and expect them to prevail, for good or otherwise.
Getting back to the immediate crisis, the Northern forces learn a hard lesson and realize they have to rethink just how they are going to drive off the Normans, and for the moment, that they had best dig in and prepare whatever ingenious defenses they think might work to check those powerful knights and give themselves a fighting chance to just stop their expansion further. And somewhat decimated at the moment, somewhat demoralized, with those English forces whose homes are still fairly far away from Norman lines not only wanting but absolutely needing to go home to tend to the crops.
Well others who were scooped up by Harold initially or might have joined up after Stamford who are from the south have already lost their homes--they are still there, though God only knows how their kin and neighbors are suffering, and trying to sneak home and humbly accept whatever the Normans are dictating there would be problematic since obviously the Norman lords and whatever English are serving them will have questions about where these men have been and what their intentions are now, it being either a known fact or a pretty obvious guess they were off with Harold, or ran away to join Harald. So some of them will desert, and either wander around as loose wild men or try to get home again and take their chances arguing with the Normans who have some very very pointed arguments on their side indeed! But others will figure they have no place better to go, and maybe if they stick with Harald's army they can go home in some style and honor someday when the Normans are finally beat, and realizing that won't be anytime soon, but maybe still hoping, in their lifetime anyway. Can the emerging and crisis-ridden new kingdom of Northumbria actually provide for these men, can they be fed by some means other than letting them run wild and pillage Harald's own new subjects, and sheltered, and clothed, and their arms kept up?
Insofar as they can be supplied somehow logistically, I am guessing some fraction of what Harold recruited remains under arms and under Harald's ultimate command via mostly English officers and sergeants (the rank names and exact roles are anachronistic, but not the general broad requirements of commanding infantry forces I suppose) and it is these men combined with many of the Norwegian adventurers Harald brought who form the garrisons guarding the southern fringe of Northern defenses in this season of retreat. Behind these lines, to the north, the soldiers return to the land, albeit now battle-seasoned to be called up as local militia should the Normans surge on north, and resume the routine business of sustaining the realm. Near but behind this front, Harald and his new English ministers ponder just what steps they should try to take next.
And in between lies a no-mans-land, which at this moment is still pretty heavily populated, only somewhat denuded of young men and some of the older who had gone off to fight for Harold and not returned, either because they are lying in a grave somewhere between York and London, or because they have decided not to come home yet or ever for various reasons. But they are still despite being somewhat short handed valuable resources for whomever can come in and claim effective overlordship over them first. At the moment prudence, in the form of William holding the Norman barons on a tight leash, chains up the Normans southward, and in the form of Harald plus his impromptu Witan combined with the bitter experience of having their asses handed to them similarly the proto-Northumbrian forces are backed up some distance north.
I think the process of a general devastation and ruining of the middle lands is already going to be underway before the north and south forces each march to meet the other in a long series of violent clashes. I supposed the land retained almost enough workers, mostly women, young boys and old men, to largely get by, but with each town having news of the dire situation that exists some people are going to start slipping off--if William can do a very good PR job some of them will slip southward, others, probably the majority of the footloose, will make their way north, and the land will be gradually abandoned--those who either bold or foolish enough to remain can probably step into the most desirable lands and do well for themselves despite being shorthanded, but they will regret it if troops from either side show up to take their loyalty oaths to their side's monarch and requisition the goods. Each side will be keen enough to protect whatever they can hold to be sure--but if they find they are losing their grip and must retreat, they won't want to leave good land and partially grown crops and goods for their foes to rush in serfs to tend to them and produce more items, so they will burn the fields and otherwise try to wreck whatever they can't grab and run with.
My first engagement with this thread before I was moved to start responding was to look at the map offered in the opening post. It did seem to me a sort of semi-natural line of defense for the north might exist, not being familiar with English geography and not having the technical ability to edit drawings on this cursed cheap knock off Windows II system I have acquired in an emergency where I ruined my older Dell Win 10 LT, it is kind of awkward for me to articulate it, but I think I was looking at the bay of the River Humber and then the hills fronting on northeast Wales to the west. Then I came to realize how very far north that was and how little it left "Northumbria" so now I am hoping we can arrive at a line considerably to the south of that!
But I do have a sort of broad sequence of events in mind based on that first map, pardon me if my attempts at verbal handwaving are not entirely clear:
1) at this juncture, after the grand northern combined army descends on a Norman partial concentration, I am guessing on the direct road to London but north of the Thames, say on that low ridgeline I think I see going diagonally up into East Anglia, roughly the line from the tip of the bay separating Cornwall from Wales proper to join that ridge, and then gets beaten up and retreats, I think Harald will call a halt and disperse groups who remain with him, mixing Norwegians and English, on a rather unpleasantly far northern "line." To assist, I tried to look up a suitable map of England, but with Bing's insistance on commercial stuff I am not sure I ought to link to it because of copyright, nor I am I very pleased with it either--curse Microsoft, curse Bing, curse Win II. Anyway, I see now I am looking pretty much at the Trent, flowing out of the southern part of the major hill range running north south in the middle, shifted westward, of northern England. To get some distance southward from the region of Lincoln and southern Lindsay so far north and close to York already, I note the light ridgeline that runs through whatever that section of English middle eastern coast is between Lincoln and East Anglia, and suppose Harald proposes to set up a string of skirmishing camps running more or less straight east from the south tip of that range to the bay between that region and East Anglia, prepared to make fighting retreats on the east north to the ridge and try to interdict any attempt of the Normans to surge on to the coast, which I suppose is the more productive and populated land, giving way as necessary to the line of the Trent itself. This is only temporary of course, the idea is to regroup, recover while maintaining a tripwire line of defense, and hopefully accumulate a force strong enough to go south again for another drive to take London.
Meanwhile despite a plain victory in fighting off the first sally from the north, William and his more astute cronies are aware they are kind of overextended already. One major part of Norman strategy of course is to build castles, something they were quite good at and which combined with their knightly forces into a formidable military machine. If he has time to build a suitable number of castles he can be confident, or so they figure at this point anyway, that the English won't be able to get far past it--they can probably figure out, somewhat inefficiently, how to besiege one castle at a time, but the garrison can hold out for weeks and months while other Normans come rallying in to pin the English between hammer and anvil, either destroying them or forcing them to break the siege and run for it. But I presume it takes some considerable time to make even the rather simple and rudimentary "motte and bailey" type of small castles characteristic of the 11th century, so William reins in the more gung ho lords, holds to a light line on the ridge north of the Thames that leads into East Anglia, planning to move, once that front line is well fortified, into the valley of Avon and Wye to the west to straighten up that line and incrementally advance northward some years hence. Small parties of Normans are authorized to go forth on sallies to see what they can grab cheaply and quickly with whatever authority William's claim to the throne countersigned by the Pope can get him, combined with personal inducements of a positive or negative nature as the case might be, but stand ready to come running back to sustain the southern line for now.
Harald's northern lot has a lot of people in it impatient to free the south, as many of them are from there and worried sick about what is happening to their various southern English homelands, but these same men have also had a taste of what the Normans can do to a wrongly balanced and inadequately sized force of infantry, so they largely heed Harald's call.
The English of course have a rather ancient tradition of lines of fortification of their own; there are earthworks like Offa's Dike to consider, and also Alfred the Great came up (along with funding a navy of sorts) with a scheme to systematically create and man town fortifications against the "Danes" as the heterogenous bunch of Nordic adventurers in Alfred's day were known. Those probably aren't just the right thing versus Normans who know a thing or two about siegecraft, but just as even half-assed mini-castles buy time for Norman defenders north of the Thames, fortifying some lines of towns in that zone between the western hills and lands south of the Trent can no doubt slow up the Normans too and again give them a choice between bypassing northern strongholds (rarely if ever a great idea to leave such a site at your flank or back) or tie themselves down trying to reduce them, buying time for the northern forces to assemble and descend on them. Somewhat like the Norman foray/raiding parties, groups of men Harald soberly judges he can risk losing are blessed to venture out southward and see what they can accomplish on the loose, so the lands between these Northern and Norman lines are falling into a very dark age of raid and counter raid, with thugs of one side or the other setting themselves up as overlords of the moment and grabbing and running with everything they can carry or drive before them when a sizeable force of the other side shows up to challenge them. This is not quite symmetrical; a lot of Harold's original army is locals from these regions and these guys will try to hold on to their old homes as best they can, and when they decide to run north instead it won't be so much like a slave raid as a commando rescue operation, but however much difference that makes to the women and elderly folk and children running north to a dubious life as exiles and refugees subjectively, objectively they are on a long, painful, hungry and sick and tiring dangerous gauntlet of a trail to a rather sad immediate future as supplicant beggars in the north.
This mutual scorching of the earth between the defense lines is going to change the overall dynamic of desired expansion for both sides. Assuming each set of lines can however hold in the short run, it buys time to strengthen them in the longer run, and behind them, north and south, each royal regime is pretty secure to rebuild England on their respective turfs according to each regime's character. The more of a no-mans-land the intermediate terrain becomes, the harder it is for captives in Norman territory to have second thoughts and run north in turn, though I suppose a trickle of such refugees would remain a thing for decades and generations to come.
One other thing I saw on the OP map, looking at the farther line along the northern lower reach of the Trent running into the North Sea, is that to its west beyond the central ridge line there appears to be a broad valley zone with two rivers leading into a bay on the northeast corner of Wales; the northern river is the Mersey. I don't know the climate map of that section of western central England, but geographically it looked to me like one could have a prosperous zone in the lower part of the valley near the coast, and the slope going east is gentle, allowing for a pastoral hinterland pretty continuous with the lowland farms, and a population native to there committed to defending themselves would enjoy some advantages setting up sentries along the eastern ridgeline, which beyond that ridge peak falls down eastward in ruggedly hilly terrain that would be difficult to send a coherent attacking force up through. If that curved wall northwestern valley could be in Northumbrian hands, with a loyal population filling it, the western part of the line drawn to check the Normans is pretty much pre-built, or so it looks to me.
Then thinking about how Harald's personal invasion force would surely suffer some attrition at Stamford even if he did quite well overall there, and the English force while large is not after all exactly a Hunnish horde either, I am guessing instead, it might be smarter at least for the moment to hand off defending that part of the north wall to someone else. Specifically I am thinking of Wales of course. Approach whomever is the current overlord of northern Wales (it is not clear to me whether that guy would also be the lord of south Wales at this time) and magnanimously grant him this extension of Welsh territory in perpetuity, perhaps including some of the hill country beyond the ridge as well, with the clear subtext that if these Welsh will just defend this land and not let any enemies of Northumbria into or through it, Harald and his successors will leave it to them without quarrel.
A clever little trick is that this more or less crescent shaped west-facing valley scoops all the way south to the latitude roughly of East Anglia's northern coast, and Harald would figure he'd be doing quite well if he managed to secure any land south of that line in these circumstances, so while he has not granted the Welsh the land the opening of the valley in the south eastward opens on to at all, it will probably be either no-mans-land, or under the Norman thumb--so if the Welsh get expansionist ideas further eastward, behold, their obvious way forward is pointed like a gun (or crossbow) at the northwest marches of southern England. Vice versa, the Normans are more of a threat to the Welsh, if they occupy this eastward extension, than the Nordies are. It might become the basis of a very long and amicable alliance with Wales permanently on the Northern side.
In the longer run, where the border finally does stabilize, will depend a lot on demographics of both sides, economics, and of course the details of military tactics and strategy. I believe if the North can buy itself time, some 20-30 years or more, to largely develop with no more drains on it than holding that southern picket line, they can over that time develop some heavy horse cavalry ability--probably remaining inferior to Norman knights, man to man and horse to horse, but also integrate them into a more balanced combined force with various anti-chivalry tactical tricks. Over time the fortifications and siegecraft of both sides will escalate in an arms race that will be something to behold, as castles, town fortifications, and field works are laid down in sketch and then elaborated.
Again I am handicapped in my understanding of the nature of the terrain on the ground, but just looking at the topographical map I suppose the north will advance over time to the ridge running north of the lowland running from the highlands of Exeter to the mouths of the Nene and Great Ouse rivers, with the Normans dug on on the south ridge, and they will fight increasingly intensively in that valley and gradually hammer out a more or less settled land border, probably along the course of one or the other of those rivers, or with one side (probably the Normans in this case) pushing their way right up to just below the ridge peaks of the other's ridgetop line. In area this division looks like a pretty even one to my eye, though it would help to have some geographic tools to verify that--in economics and demographic potential, the North is clearly going to be trailing behind the south. Though for what it is worth, I don't think it is too irrational, sentimental or romantic to suggest maybe the way the North evolves socially would be measurably healthier and more balanced politically, and might have aspects favoring vigorous self-development offsetting the virtues, in their grim and inhumane fashion, the Norman more terroristic approach has along those lines of hard nose to the grindstone incentives. Also, I am hoping that by gifting the Welsh with the strategic concession of that modest zone of the northwest, the north kingdom defuses quarrels with them, wins them over as friends and allies, and thus secures its sea lane control of the corridor into the Irish Sea and northwest of Scotland to indeed consolidate the Scottish Isles and the other coasts of the region.
Having Scotland as an enemy would not be a good thing, but I read what did happen OTL as an indication that here too, Harald can lay foundations for essentially friendly and cooperative relations that might lead over time to Scotland's lowland core drifting gradually and peacefully into Northumbrian consolidation; then even if the Highlands have resistant elements these can be either conciliated or crushed.
The upshot then would be essentially a Greater Scotland coming down farther south and with its center of population more southern and English.
Meanwhile, if Wales is a permanent friend of the Northumbrian realm, the Nords seem likely enough to assist the Welsh in holding their own versus Norman expansion. Wales thus remains on the map, its people speak Brythonic.
And what about Ireland? Northumbria as I am now styling it has an outlet onto the Irish Sea from the get-go, Ulster lies near to hand. Over time I would think the Nord kings (along with various factions of subjects acting privately on their own hook) would poke their nose into northern Irish affairs, offering support and help to this or that petty coastal kinglet who seems "buyable" in this fashion, and gradually start to build up ambitions about knitting together an integrated set of Irish dependencies to run in parallel under a common scheme of development.
And then, seeing this foreign cherry-picking proceeding, rival Irish interests would see their way toward more active cooperation with each other, versus the foreign puppet masters of Northumbria, and a counter-federation against Northumbrian hegemony might also evolve, thus Ireland would perhaps have two evolving nation states emerging on its soil. Or not of course, OTl the English Pale failed to catalyze such a federal union after all.
