Pretty cool game, I had fun playing it. I particularly liked the power-ups and the variety of enemies. The music was also well done and I liked how you get to see where the next wave will spawn from.
However, after a while I felt the gameplay got a little repetitive. Apart from the powerups, there were no new features introduced as the game progressed. Maybe you could give the player one tower to start with and have the others unlock as you progress? Also the main menu has no quit button and once I started the game there was no way to return to the main menu except by dying.
Still, nice concept and it has a lot of potential.