It's a big building, for sure. I'm using the Pegasus Hobbies Gothic City Building Large Set. It's a good number of pieces that pretty much let's you build what you want. More importantly, it's incredibly easy to put together. I added the flooring myself. I'm considering adding a bit more to the rear wall, but not sure yet. I still have quite a bit more work to do, but wanted to let the glue set.
I wanted to make sure that a Rhino could make it into the ruins from at least one side.
Hey, look, Dreadnoughts can get in as well!
Looking down from the top. Obliterators on the roof, rawr!