Not bad, but needs work
You've got some great concepts here, and every level does a good job of making you have to do new things, but the one really big beef I have with it all is that it's SO SLOW. Even on the highest setting, these bullets are traveling ridiculously slow. There's also a bit of frame-rate issues whenever I'm going from one screen to another. The most glaring example of this is going into the title screen from your opening logo. I can run Crysis on this PC, so I doubt it's my computer.
On top of that, the other thing that bugged me was the fact that even when I'm aiming directly at the other tower, it still takes a bit for me to hit it. It realize it's a physics-based tower game, but unless you use those physics in a way that doesn't frustrate me (like having to bounce shots off of walls or something awesome like that), you may as well just cancel out enemy shots with my own as soon as I hit them instead of making my new shots fly all over the place.
Other than that, this has potential. Keep tweaking it, and you may have something here.