Nice game! Those extremists are relentless, I can only make it through a few waves. Also, I can't seem to upgrade "towers".
To be honest, I have the same problem. My teammates managed to win the game but so far I always lose during the last wave.
What happens if you try to upgrade the towers? Is it possible that you don't have enough money to upgrade? (The Paperboy upgrade currently shows the wrong value, it's actually $200)