Not bad
It's pretty cool as a remake, but not so good as a modern Flash game. Unfortunately, Flash surpassed Atari ages ago...
I like that you took the time to make or find some actual sprites for this instead of stooping to geometry. I <i>hate</i> geometry.
It was a bit weird when the aliens got close and started plowing through the defenses, but I guess I don't know how that could be fixed.