Castle Crashers is possibly the best way you can spend 15 bucks.
These graphics are pretty good for what they are meant to be. Not spectacular and not bad, but definitely good.
Gameplay 9.5/10
The gameplay is simply amazing. Emphasis on "simply". The combat is very simple and very addicting. Just simply mash 2 buttons to perform combos. To make it better, magic is implemented seamlessly into the combat to add some flavor. Another aspect of gameplay worth mentioning are the boss fights. These fights are amazingly fun and will be fun the 100th time you play it.
Extra Modes 6.5/10
Arena is O.K. in my opinion, but volleyball is where the points are lost. The good thing is that volleyball can be avoided almost entirely except for one spot in the story mode. My advice: just don't touch it.
Multiplayer (Offline because I haven't tried online) 9.0/10
Castle Crashers is even more fun with a few friends. It's easy to simply jump in in the overworld and start playing. Playing with friends doesn't do much other than make the game more enjoyable overall.
Sound 10/10
After playing this game for a while, I have fallen in love with the soundtrack. The music always fits perfectly with the environment and is engaging and exciting usually. The sound effects aren't bad either and, surprisingly, don't get repetitive.
Longevity/Replayability 10/10
This game has tons of secrets to uncover. There are a total of 25 characters, 69 weapons, and 28 animal orbs iirc. Also, there is an Insane Mode that ramps the difficulty up significantly. Playing through the game as different characters never gets old and provides great variety.