I nerfed both the slow tower and the fire tower last minute, but I didn't nerf them hard enough. I had planned to have units with resistances and weaknesses to those towers but I didn't have time.
And yeah, the scaling of the AI is absurd in the end game. When I was testing I never got that far. I didn't even complete my first run of the game until after I had submitted, at which time it was immediately obvious that the game had balance issues. It's saved slightly by the fact that the AI is only allowed 3 actions per turn (building or upgrading towers), so it just winds up with more money than it can spend by the end of the game.
I like the idea enough that once the jam is over I might polish it into a better state.