The look and style of the game are picked nicely.
I probably would have wished for some mechanics that supported the theme. The towers only shot at the enemies, which is kind of what I would expect from a tower defense game anyway. Also I think you could have played with the perspective a bit more. It still felt like the "bad guys" were coming from "my side", which also made it feel a bit more like a traditional tower defense game.
Still, I don't want this to sound bad. It's not a bad game. I did have fun playing it.