I have a number of things to say about this game. First of all don't make enemy ships appear from beneath - it's annoying. Second, make sure that rocks damage enemy ships as well should they collide. Also do not assume that the keys X and Z are next to each other on every keyboard (see keyboard layouts). Enemy ships that collide with my ship should receive equivalent damage. I expected the big square-like ships that appear on some levels expected to carry powerups. Some bosses are difficult to fight because they take up most of the screen space, so you can't move further away from them. Perhaps the enemy ships should also receive damage by cross-screen lasers (friendly fire).
Overall it's a decent game.