Amazing use of the theme, really unique. I love the concept - though I think it needs a bit more polish. Towers don't feel like they're doing anything at all, you want the first wave to be filled with enemies that die in just one hit so you can get a feel for things. You should also make the bullets MUCH bigger - it's really hard to see them!
Otherwise, though, it works well and plays quite differently to a normal tower defence game. Great job!