Cool game! I really like tower defense games, and I like the idea of two dimensions defending the same thing. I cannot for the life of me figure out how to unlock the $100 demon tower, but if someone else does, please tell me lol. I tried filling every square on both sides with towers, getting a very high score (300k+), placing a demon tower at random spots you're not supposed to, and clicking all over the place.
Anyway, great job!