First off, fantastic game! I found the first 2 campaigns pretty easy, and I'm having a blast playing the Carl's Crazy Funhouse levels (my personal favourite so far was the wizard pinball).
Anyway, I'd like to point out a little flaw in the level 21 Wizard Salute. At the second crystal the wizards can walk by, if you place a Gargoyle to adjacent to the left of it then use an invisible wall to stall the first wizard, you will have enough time for the Gargoyle statue to refresh, allowing you to place another gargoyle above this second crystal. By carefully replacing this second gargoyle (it will get destroyed once in a while) you can freeze all these regular wizards indefinitely with no way of breaking them out. Using this strategy I was able to Wall Press the entire lower alley, killing all the Epic Wizards, but I still couldn't win!
I think an easy fix to this would be to have an Epic Archer/Wizard come up the top before or after the Epic Wizard salute part, as he would be able to take out that second Gargoyle without getting indefinitely frozen. Alternatively you could provide some small defense, like the bomb or floor spikes, which would allow the player to take care of this issue themselves (though it could upset the balance and creativity of the level.
Again, Thanks for making this awesome game. I had my eye on it and usually wait for games to go free (as most do, and even your geoDefense [another excellent game] and geoDefense Swarm [downloaded but haven't played] have, although it took a long time ;) ) But at the opening sale I just had to grab it and had tons of playing time and am thoroughly enjoying it! I can't wait to keep playing this and move on to the Gauntlet :D