A really cool premise. I could see the game being really fun since you'd be constantly removing and replacing turrets. However, even with one turret on the map, it shoots very slowly. Creating any more turrets means they won't be able to shoot enough at all. I only got pas the first wave on one map once after taking a lot of damage.
If rebalanced, I could see the game being really good though.
There is a bug where, if you lose, you can't go to the same map. You'll instead go to the title screen.