Great game!
At the beginning it was quit diffecult, but afther level 6 it became easy as hell! No enemy could reach me anymore. Now i'm at level 38, and my base don't even need me anymore! :( more than 400 tower guards, and all the upgrades you can get! A tip: if you're going to update it, or make a new sort of game, make a Anti-Air site and enemy helicopters and stuff. Then it might become more......diffecult :P