I like this game, but it needs balance- right now I put it on a second monitor, and it kinda plays itself. After you build all the turrets, and one healer, my guys are just toing a total massacre! Oh! When I was writing the last sentence the game finished (after 665 seconds and 17500 points).
It was more fun towards the start, when the placement of the buildings, and what you placed mattered. I lost a couple of games at first, because of that. Then, after I got initial placement right, I could focus on building more, and once I had all the places I was unstoppable, and it became a waiting game.
The graphics limitation are used well- you use only two circles for a turret, but because they are off-center you always know where they aim! Particle effects when all your guys are red are cool too! Really sell the "maximum power"! Pixelated font fits the retro sounds and simple graphics.
The sound is ok, but some music in the background wouldn't hurt.
As for the theme, I think you could do more with it. Like if there was some space anomaly, that would limit where you could build at a given time?
And the last thing- you SHOULD be proud of this game! Read the comments, figure what you did right (graphics, gameplay at the start), think about what you could do better (sound, theme), and see you on the next jam :)