I like spoomonkey's idea of a secret room. I envision a statue of a monkey and pulling the tail makes a wall spin around.
Now that I think about it. Walking into his house and seeing statues of monkeys would be pretty funny in itself.
The courtyard idea is very cool. Having a hot tub that no one could see is very cool and now is on my wish list. I think finding a square house might be problamatic, so maybe I can find a house that with some remodelling could become that.