Not bad, good concept.
I felt like it was much too easy. You could definitely include an upgrade system of some sorts (they seem popular nowadays) and/or increase the on-screen up grades. gun didn't shoot far enough and aiming was a little loose. music was good, more work it could have been better. add more levels, don't use a static terrain and include a reward (upgrade system) at the end of each wave and i think you'd have a front page game right here.