I can't fault your work.
Very interesting game, but it fails in two places.
Number one: the game is very repetitive and gets boring quick.
Number two: You can keep shooting the enemies after they have been destroyed. Looks like a coding error.
All in all however, the graphics were good, the sound was excellent, and the level design was innovative. As previously stated however, it's really repetitive. I'd recomend not moving up and down and concentrating on having interesting enemy movements.