Brilliant!
I am loving this game! I will be playing for hours with this. I'm honestly surprised to see that this is the first game using this twist as the basis. I've seen the reversal in a couple levels of a different Tower Defense, but it was just that, a couple levels. The game is challenging, but so are regular tower defense games. And aside from the gameplay, the art and the music is very good, too. Also, the Hotkeys were a very good idea.