Your challenge is simple: Swap the order of the Crusader and Turkish conquests of Constantinople.
- Neither conquest has to take place exactly in that year, you get a half century leeway in either direction.
- The Byzantine Empire should remain in a sizable and recognizable portion after the Muslim conquest, as it did after the 4th Crusade.
- However, a realm independent of the Catholic conquest should not exist (it may linger for a few years as Trebizond did).
- The new Catholic state should be comparable to the Ottomans in longevity.
- Neither conquest has to take place exactly in that year, you get a half century leeway in either direction.
- The Byzantine Empire should remain in a sizable and recognizable portion after the Muslim conquest, as it did after the 4th Crusade.
- However, a realm independent of the Catholic conquest should not exist (it may linger for a few years as Trebizond did).
- The new Catholic state should be comparable to the Ottomans in longevity.