Castle Crashers is a throwback to the old days of Double Dragon and Golden Axe. You choose a character (there's very little variation between them) and set out on a quest to rescue four kidnapped princesses. Your quest takes you everywhere from a crazy cyclops wedding to a volcano to an ocean full of ninja pirates...
The graphics are great, they nailed the cell shading. Animations aren't as impressive but they're meant to be simple. Controls are easy enough, you've got weak and strong attack, use item, block, etc. There are combos to do but typically the game comes down to button mashing. The sound is alright, not all that memorable.
Gameplay is what you're after here, and it's what you'd expect from a game of this type. Go solo or join in with some friends and hack and slash your way across the landscape. You'll find new weapons, you'll find pets who will help you fight or find items for you or whatever, you'll ride deer... it gets crazy. The gameplay is a lot of fun and as you progress through the game you'll level up your character to make it stronger. While there are a ton of characters to unlock, the issue is that they all play almost identically. You can level them all up but I think you'll get bored of it long before you get far into that process.
There's a few negatives. First, the game gets repetitive. Hack, slash, level, repeat. It's fun but there's really no special way you need to play the game to tackle a boss, just keep whacking away at them. Finally, the humor gets really juvenile at times. Driving a diarrhea-powered deer is just stupid... and gross. Lots of that sort of thing throughout the game.
Overall Castle Crashers is fun but it's nothing groundbreaking. If you can get a few friends to join in online (or on a single console with you) you'll have even more fun.