I've tried, honestly TRIED to get past Katy Bar the Door on Gauntlet. It's hopeless. My hands just don't move fast enough. I haven't tried very hard on CFH2, just gave up on the first level. I did manage to finish CFH1.
I would like to suggest some kind of purchaseable or unlockable thing that would help with the Challenge levels, even if it means that using it would not help my score in GameCenter. I don't really care about my score on GameCenter (most of the time - it's cool to know that I'm in the top 5% or whatever, but I don't get my ego wrapped up in it).
Various ideas, any one of which might make it possible for me to get past Katy Bar the Door:
1. One empty slot that could be filled with whatever I have unlocked (I don't think any of the challenge levels use max number of slots).
2. Ability to equip one powerup before the level starts. Possible powerups:
* Start level with 200 extra mana
* An extra full second for each wave (so more time for mana to gather, more time for defenses to recharge, more time to THINK)
* Recharging of defenses takes 90% as much time as normal
* Mana pools and mana crystals/crabs spawn mana a bit faster
I suppose that the empty slot from #1 might be considered a powerup too.
3. For campaigns that are nothing but Challenge levels, a way to SKIP a level, so that I can at least try my hand at the rest of Gauntlet. This would be particularly frustrating if I had paid $1.99 for Gauntlet, then never managed to finish Katy Bar the Door which is very early in the campaign. Since I got Gauntlet free, I can't complain, but other people might be in the same situation.