That's great!
Well, lets see... I would either change the fire key to be on the keyboard, or make it so that when you click, the direction you fire in is based on which side of the screen the mouse is on. That way, you avoid the confusion I had altogether. It might also be nice to extend the fire range a little, the only way I really started defeating aliens was to rush right into their faces and spam click.
If you really want to expand upon the concept, I think it might be interesting to give different aliens different movement patterns and behaviors. Perhaps some of them could have longer range weapons but move/fire slower? Or, perhaps certain aliens have a weakness to certain weapons. That sort of thing ^^