As far as gameplay goes, it doesn't seem to particularly balanced. I played the first two campaign levels and they just start way too slow. I would think that a good tower defense game might want to give the play a sense of urgency throughout the game, and not just at the end when their health seemed to jump up quickly.
Also I might want to spend a bit more time on polishing up the aesthetic of the game so that it is more coherent. For example, on the main menu the title of the game is artistically stylized, but the menu buttons are just Arial font on a sort of ms paint looking-shape. Also, what is the background? What does it have to do with this game?