I like the idea of a strategy game where you control both sides. The invaders have very cute but creepy designs, I dig it! While the palette-swapping works fine enough, it might have been better to also give them unique faces. The weapons were a bit harder to make out. I gotta agree about making the turret bullet-based rather than timer-based. A level structure where a certain score would be needed in each level could give it a real gamey feel; at the moment, it's more like a tech demo.
Interesting concept to be sure!