Not bad, not enough depth to keep me playing very much though.
A few things:
1. There are some lag issues. Not sure where it's coming from since there are relatively few things on screen at any given time. For some reason i have a feeling that it is coming from the way you generate the footsteps. You don't need to generate them constantly.
2. You're not rotating the bullet image. When you get past the first upgrade, you can tell that the bullets are firing at different angles, but the image will only point in one direction.