Great game! My biggest issue is with the turret placement, every time I wanted to shift or switch out my stuff my base seemed to bulge awkwardly and involved the movement of too much stuff. I understand that some things are bigger than others but I had issues simply rearranging the same things in the same space.
Kind of not cool to have in the "physics pack" achievement when it doesn't have legitimate physics in it. Rarely does something run into another something and instantly stop dead.
Lvl 6 is indeed impossible, I've watched people cheat with infinite money and tried it myself. You just can't kill the air in the later stages. If you have problems with ground guys, build a long maze that has a blocked exit near the front and an exit at the end. When the ground is about to make it out of your maze sell your tower blocking the exit near the beginning and block your original exit. They will turn around and go back through your maze :)
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