I'm a big fan of this game, but one suggestion, it gets kind of samey once you reach level 15 and every level after that is the same enemy, I'd suggest maybe repeating the order of enemies, so wave 16 would be the enemies from wave 1, 17 is wave 2 enemies, and so on
It'd also help distinguishing between waves you're trying to run early vs. waves you know you can handle when you're trying to get a little extra cash