For a long time I've been trying to figure out how to make it possible to build variated medieval cities without having to build each building by hand. I came up with a system of modular design where each part is it's own object. This can for example be a window. This window also contains multiple parts, like frame, window and boards. Then I can export the object into Unity3D and hide various parts of the object as I want. I can also easily change the material of one part without having to build new objects for it.
It's a bit difficult to explain it and if I ever get around to it, I might make a tutorial.
This might not be a very good or efficient way, but it's one way to do it.
I'll upload the files here if anyone wants to use them.