It would be cool if you added astroids to make it a little harder. Like at first they're really small like twice the size of the shots and real easy to kill but then they get bigger and bigger and take a lot more shots to kill. Same with the damage they do to you, the little ones don't hurt you much but the big ones hurt you alot. Also, when you shoot the big ones the burst into smaller ones.
I agree with Ipfansether about the missle caps and i'm think 8 would be good for when more and more enemies come out they won't be too easy to kill.

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