It was really fun for a bit, I feel like it got exponentially difficult all of a sudden. I got stuck on the next to last level, and could only beat the first two levels on the more difficult setting.

P.S. Maybe I'm just old
The thing with TD games and the like where you have a lot of options, you can often hit on a strategy that is effective but not expected/known by the devs and so the difficulty curve of the game seems totally out of whack to you. Maybe that's what happened.

To me this game was wicked, it's obviously not a new idea but polish is king, both on the cosmetic side and the actual game dynamic side, and I'd for sure rec kingdom rush to anyone