I see you put some effort into this, but it could be better. First off, the ship's controls are sticky, sometimes you can move diagonally, sometimes you can't, which makes it really hard to avoid enemies. Secondly, I believe that the enemies are too big for a screen so small, I couldn't even get past level one. And finally, the collision detection seems to be a little off.
I'm sure with some tweaks and a bit more effort this could be an excellent game.