I think I've told you before, but I'll post it here anyways. In more detail... ;D
It would be a huge, gigantic mansion that has twisting staircases that moved, and some would wobble, and some would throw you off onto your head. Tricky doors would lock behind you, and some windows would be dusty. Some rooms wouldn't have any doors, but plenty of windows so you could look inside them. There would be a few floors you couldn't get to unless you had a grappling hook or something.
The rooms would be full of characters from my favorite books, or my favorite ideas for books. Different stories would have their own room that fit them. Like Count D would be in his pet shop, and L would be in his high-tech room full of screens and speakers. Kendrick would just walk around at his leisure, because he doesn't quite have a story. Lots of bats would be there, too. They would ask you confusing questions.
One of the rooms would have an extremely high ceiling and bouncy floor, just for fun. :3
Beyond Birthday would follow you around and stare at you until you felt uncomfortable. He knows the quickest way to get to all the rooms, but he won't tell you unless he wants to.
If you can find your way outside the mansion, there's a little rosy garden that's laid out nice and pretty, but just beyond it is an endless, gloomy forest full of creatures that will shock you and tell you scary things. The sky is always overcast.
Well that was fun! I'm gonna try to find some music now.
Himitsu >> Listen to the whole thing, it gets weird. n_n It's from that awesome anime called Death Note.
Long Past Gone >> This is for the more calming rooms and the garden. I got the song from this crazy game called Braid, where you control time and use it to solve puzzles.