I didn't unlock any characters. I think the time in rounds it takes to upgrade "new" upgrade options makes it not worth it. I can imagine Assassin being eventually useful, but given you made upgrading targeting/accuracy a liability, I don't trust you to be considerate enough and it's too much effort and focus to try to figure out a simple tower defense game. I only came to check out the comments one last time to see if anyone had gotten so much further than me and to perhaps find out how. I'm not interested anymore. I don't understand why a dev would make a game that was so difficult and unrewarding to play that it actually turned into a bit of a bummer once I'd invested enough time and energy into it.