Do you want buildings that models can move into, or basic sight-blocking stuff?
4Ground makes gigantic gothic city buildings - but they're a "facade" line so that you can use balconies etc. but no interiors (unless you build them yourself). By gigantic, I mean...some stand around 25-35" tall I believe.
None of them would look out of place on an Imperial world. I've shown a few below:
They're prepainted
MDF and cost a good bit (you're getting a Warlord Titan worth of material for some of these kits).
For larger buildings with interiors and useable space you'll definitely be looking at scratch-building using foam core.