Hey, I wanted to throw out a suggestion for improving balance and expanding the game with absolutely minimal dev effort:
Currently the game basically ends after 12 turns or so if successfully cheesed (buying money towers and minimizing the progress per turn so you can build up a huge army by the end-boss, around turn 16-20).
You obviously don't want to put effort into making new units just for people who are abusing the game, but you *could* take a page from the 80's/90's era book and just make color-swapped massively boosted version of the existing enemies, to come after a few rounds of the shielded knights (maybe turn 12 or so?).
You wouldn't even need to worry about balance too much, because anyone still alive at that point has pretty much mastered the game, and such units could be avoided by winning more quickly.
The point of this would be to make an in-game incentive to win quickly, rather than just guaranteeing a boss-win by taking your time.
I think it would be really fun to start running into 8/8 grunts or, eventually, 15/15 knights, or things like that.