Here's the challange the way I play it.
Premium Content
Normal Difficulty
Build no towers: Reinforcements, Rain of Fire, Lightning only
No upgrading towers provided at the beginning of the level
No Elves, Sasquach, or Sunray
I'm fully upgraded, so you have the advantages of Cataclysm, Throwing Spears...ect
Basically, you can spend no money. I found it helped me learn how to better use my "specials", and it's a lot of fun. I haven't tried the Heroic or Iron stages yet, but I can beat the first fives stages, and I'm close to beating the Citidel. You really have to protect those Imperial Gaurds (especially from the Shadow Archers).
BTW: I've also nicknamed this the "Broke Ass King Challange"
Enjoy!