Well, it looks like lots of people are starting to make guides on the forums, so I decided I should post some of mine here.
Instead of rewriting them, I'll just link you all to them.
Castle Crashers - The Complete Basics Guide: http://steamcommunity.com/sharedfiles/filedetails/?id=498713480
Castle Crashers - The Best Ways to earn more XP Guide:
http://steamcommunity.com/sharedfiles/filedetails/?id=491525993
Castle Crashers - All Animal Orb Ratings Guide: http://steamcommunity.com/sharedfiles/filedetails/?id=484753116
Castle Crashers - Better Keyboard Controls Guide (Steam Only):
https://steamcommunity.com/sharedfiles/filedetails/?id=481662370
I will expand this as more guides come in the future!