I personally love this game and think it has amazing replay value, because you can beat it with several characters and unlock new characters. My favorite knight is the fire guy, and my favorite overall character is Frenchy's minions. I killed the cyclops boss with his magic before he even took a step. :P
I think the most important skills are defense and magic. These help you level up fast, because defense you die less, and magic does lots of hits and damage. I don't suggest getting strength until you have very high defense and magic. I suggest getting defense full, and magic as full as possible until about level 25. Then get strength. If you get strength to early you will take less hits on enemies, therefore leveling up slower. SO defense is the most crucial always. Strength is very important when you try insane mode though. I suggest having full defense, and starting at level 25 level up your strength only until you reach level 35. Then you can attempt insane mode.
Well I hope I didn't bore you to much, so yeah talk about Castle Crashers here. And if you didn't know, Castle Crashers is a downloadable game that cost $15, or 1200 MS Points (Idk what it is for PS3), and is made by newgrounds (boo)
My favorite knight is the red one. I think his lightning magic is the best. I'm currently trying to get him to lvl.99
Second favorite is probably the flying black winged guy that you get in the bad guy DLC. His attacks are pretty cool, and he looks awesome. The giraffe is obviously the best pet, since it gives you extra exp.
The game is great though, although nobody plays the mini game online :/