Nice change from the usual mindlessness which the mob (see below) demands. Good job. I like the graphical style, reminiscent of old fantasy strategy games like MOM.
A few things I'd suggest:
The enhancement towers say they increase by a certain number of points, but there's no way to tell how much these points really are with no other references to points on other towers.
Also when you can't afford an upgrade, the $ cost is too faded, it should be slightly more readable.
 "Mechamonkolo"){kind=icon}
 "Oh, the Huge Manatee!"){kind=icon}
 "Pel"){kind=icon}
 "A Short Walk"){kind=icon}
 "Pet The Kitty"){kind=icon}
 "Neuron"){kind=icon}
 "Super Mario Defense"){kind=icon}
 "UPGRADE COMPLETE!"){kind=icon}
 "How to Raise a Dragon"){kind=icon}
 "Friendly Fire: Blood and Gore"){kind=icon}
 "Sparks and Dust"){kind=icon}
 "Little Wheel"){kind=icon}
 "LongCat"){kind=icon}
 "Frontline Defense Beta 2"){kind=icon}
 "GemCraft"){kind=icon}
 "Death vs. Monstars"){kind=icon}
 "Gears"){kind=icon}
 "Tidal"){kind=icon}
 "Super Star Balls"){kind=icon}