This is ridiculous. More than half of these "challenges" aren't challenges at all. It's all based on luck. Challenge 15 is a perfect example. 95% of the time, you're killed on the first turn. That's ridiculous. That's not challenging, it's stupid.
Honestly, I must admit: These baddies are scaled perfectly. I don't think I've ever had a game where I didn't have to sell things and leak a couple in order to get a tower that could 1-shot mobs. I'm impressed.
I have to admit, I love this game. Not only does it take a bit of thinking to beat the levels, but they each provide for a bit of creativity, allowing you to change a level around a bit if you want to make your own sounds, which I like a lot. 5/5 and favorite. Great game.