Very nice idea. I liked the story behind it. The graphics are simple but polished. Sound design is great.
Steventus has already said mostly what I think so I won't repeat it. But I want to emphasize his point on feedback to the player. For me that was the biggest issue. Sometimes destroying a turret would take two shots. Other times, it wouldn't die, no matter how much I shot. And honestly I have no idea why. It could be the bullet collision/ ray cast wasn't detected (a bug). It could be that the turret was armored from that side. Or was it invencible when the light was shining on them? If I had to bet I'd say it's the first option but I don't know, and I think that is the problem.
In any case, good job on your game! :)