It uses the Bullet engine and I'm actually quite surprised that I bothered to make a relatively polished interface for placing the tiles. I was somewhat less surprised with the fact that it didn't compile after sitting on my hard drive for a few years. It turns out that there were some incompatible changes in Bullet since then, so I had to make a few modifications to make it run with recent versions.
If you want to play with it yourself, here's the code (be warned it's ugly inside, though).
