Very good base!
I love the controls!
What could enhance it:
- adding some feedback when the boo... err... the ennemies are taking damage :D
- handling the borders differently maybe? Just losing when we go outside is not that great. You could either teleport on the other side (Asteroid like) or add some bounce?
Otherwise, really nice game, I'd love to see enhanced versions (sfx, music, menus and so on...)