This game is a great piece of work. Challenging and makes one put thought into their defenses. Towers are easy to understand, and some are useful not on the max level (I.e. The cryo cannon). That said, its hard to get to the final upgrade, they cost sas dollars and a lot of in game cash. This makes them almost mystical, and otherworldly powerful. I bought one, but for some reason, I cant buy any more. The game freezes, all of my towers disappear, and I cant interact with anything at all. Granted, these upgrades arent needed to play the actual game, but it serves as a sense of progression! A permanent unlock! Please fix this glitch, in an otherwise fantastic game!