I think the game is just slightly better than unplayable. I can get through the first few levels, but eventually the fact that the towers miss combined with the speed of the fast enemies is just impossible to overcome. i end up with 30000+ money and 20+ towers, but i still die to the same wave of fasts that just seem impossible to hit even with fully upgraded towers of all 3 varieties.
at first i thought it was too difficult, but after messing around a bit i have found some strategies that work. It takes a bit more playing around with the tower types to find a combination that works, but once you find one it works for most levels perfectly.
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