Indo-European is a linguistic term; very little architecture exists in Bronze or Iron Age Europe that has anything to do with those civilizations in Persia or India...
Celts and Germans loved to build with dirt and their natural environment. Think Hill Forts and Hobbit holes. But in terms of urban living situations, those two things aren't of much use. Nothing beats a good wooden shack with thatched roofs, though! To get large, landscape dominating buildings in Europe would be hard without the Romans, however; anything I can think of would really be more considered a monument.
With no Romans means a stronger Greek influence in western europe! Maybe base some of your ideas off of that and modify them?
I'm sure there are more knowledgeable people on this board than myself on this subject, however.