Oh, that's a really interesting concept. It took me a while to get how to play it, but when I did it was really enjoyable. It's some kind of a strategic shooter.
If you upgraded it with some variety of obstacles/enemies/powerups it would be really nice.
I like your particles and screen shake, however I missed some HP indicator as I didn't know when I was going to die. If health regenerates then you could think of something that is used in Gears of War, so some kind of shader that makes screen fade or bloody and when it regenerates it dissappears a bit.
Music was really simple, but as game requires a lot of focus I didn't even felt that, so that was good.
Nice work :)