I forgot to comment since the update...
Well, now it's getting really difficult, because the aliens drop down at each side, which is much better. Everything works well, except that when I make a change from the menu, it doesn't work until I restart the game. Alors the ship can cross the alien when they're down the screen without getting hurt, which makes me think they're some kind of immaterial aliens which can sometimes be materiel because they can destroy the turret.
If you want some idea you can make some kind of 'upgrade complete' (it's a flash game), but with space invaders. But only if you're bored. :-)
Otherwise nice work!