i have made an Italian cathedral myself. I found that things worked out easiest if i did it in pieces in seperate levels and saved the pieces as prefabs, then assembled all the prefabs together to make the final result. Compile times were about 100 times faster doing them a piece at a time as opposed to all in one level.

I would suggest doing one tower by itself, then break the main building up into sections (Nave, Asp, etc etc). That way you get faster compile times and you only have to make one tower, Nave, etc. instead of two. (Make one tower as a prefab then import that tower twice into the main level.)