more like zombie than alien. Missing many item to make this a good game but just concentrate on the basics. I did find a glitch, if i stand right under a building the zombies will move below me but not get me. An idea would be to make it so that the aliens explode when they get close to the player so you dont have to make them touch the player to do damage.

Now you can walking through enemy. Try use hitTest to avoid this. Other things would by step by step, interface, next levels, upgrades.