This is a really cool concept! I like the heavy black outline style art. Was a lot of fun trying to dodge past my own bullets to get the scraps.
I like how you can influence the enemies by pushing them around and try to get the enemies stacked up between you and the turrets.
I think you should be able to change your tower selection before you actually place it -- the money could be paid when you place rather than when you select.
The lock on turrets are really strong compared to the others. Obviously the tradeoff there is that if you get hit by it you lose a lot of health, but you can just stack them all in one area so that it's easy to avoid. The octo turret doesn't really do much by comparison -- if you buy an octo turret on the first wave it'll barely kill any of the enemies.