This is very cool! I'm always a fan of getting a lot of mileage out of a small play area and/or set of pieces, and this does both with aplomb. If I had one suggestion it would be that in levels 7 and 8, I had to switch colors so frantically it was easy to lose track of which walls were or weren't solid: maybe if unsolid walls could be partially darkened, like half-black-half-orange? Otherwise everything was nice and cleanly conveyed. :)