need shortcut keys! Enemies are too easy. To balance it out make the enemies harder and the upgrades 5 times more effective with each upgrade along with increasing cost. Also you should have decrease the maximum upgrade on the towers to encourage the use of better towers. (e.g. 5 max upgrades)
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 "Rotatix"){kind=icon}
 "Frontline Defense Beta 2"){kind=icon}
 "Whiteboard Tower Defense"){kind=icon}
 "Gravity Master"){kind=icon}