Image from Castle Crashers website.
It's very cutesy-cartoonish with exaggeratedly short knights and exaggeratedly large weapons (in comparison to the size of their bodies). It's pretty fast-paced action with some cartoon gore. Although not what you would think of in terms of "hi-def," the graphics are detailed and flow really smoothly on the big screen. The music is a mix of techno and action-movie-theme-song. It really gets your adrenaline pumping as your fighting off the bad guys with swords, battle axes, maces, bow & arrows, and magic. You can play up to four people at the same time.
Warning for parents: In addition to the cartoon gore, there are some pretty silly but gross elements. The authors of this game had some sort of obsession with animal poop. The forest animals in the background (owls, deer, bears) get scared of the battles and leave poop behind them as they run away.
You can download the game for about $12 on the X-Box Live Arcade.
Here's a trailer for the game: