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Way too easy. I actively played until about level 150, then I switched to another tab, and stopped paying attention. Right now I'm level 450, and the bugs still aren't reaching the bottom row. A set number of waves would be better. that or make it more difficult more quickly. It's decent, but could have been implemented better.
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It's a pretty solid effort but needs some more variety. Here's where I got hung up. Are the bugs any different or more resistant to certain towers than others? Four towers is not enough variety to hold a player's interest for hundreds of waves. The upgrade limits were a little misleading...I only get a certain TOTAL amount of upgrade? Some indication of how many upgrades a tower has left would be good. It's pretty fun for a basic TD game and doesn't seem broken...though maybe it's a bit easy.