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Yep I had the same issue even just during the 8th wave. It's mainly due to I didn't limit the money to 999,999 and the upgrade to a fixed time (maybe you can upgrade a tower 10 times), and the upgrade increments algorithm is shit. Mostly because I didn't get the change to beta test this myself since I just went no sleep from midnight to morning, worrying about the titlescreen, music, and sound fx & stuff, not minding the important stuff like this. Anyway, thanks for playing  2nd game! :D If  you have any suggestions, let me know :)