Greatest Arcade Game to date!
Castle Crashers' main feature is the story mode, which is intended that you play with 1 or more other players. Playing by yourself in the main story is fun, but you won't get as much fun out of it as you would when playing with your buddies.
The game does have RPG elements, where you attack enemies to get XP. Rather than traditional RPGs where you have to KILL enemies for XP, all you have to do is ATTACK enemies for XP (the amount of XP is measured by the amount of damage you do). The highest level you can get to is 100, which takes a pretty long time.
There are also two online multiplayer versus modes, both of which are very well done. One of them is called "All You Can Quaff", and the goal is to go through a series of fruits before your oponents. The gameplay of this game is just simple button-mashing (hitting both the X and Y buttons simultaneously), but when in a game with other "experienced" players, your whole arm will be aching. This mode is really fun, but not necessarily as fast-paced the Arena Mode.
As you can guess, the Arena Mode consists of various versus matches. The main "Melee" mode, where 2 to 4 players duke it out with whatever characters they have, is surprisingly intense. After fierce fights with other players, I often find myself with the post-adrenalin-shivers. This kind of intensity can level with that of even Gears of War or Halo's multiplayer.
While I consider the Melee mode the most adrenaline-rushing, there are other really fun modes in Arena. "Quick Draw" is where 2 or more players go at it, in a similar way to Mele, but strictly using bows and arrows. Theres also a Treasure mode, where players dig for treasure to get the most money, and Beefy mode, where players race to take bites out of sheep in order to grow huge and deal as much damage as the can to the others before their time runs out.
Getting back to the Castle Crashers mode (the 1 to 4 player co op mode), there is tons of replay value in this part of the game. While the campaign mode can be beaten in a matter of 1 to 3 hours, it offers tons of replay value. With over 20 unlockable characters and dozens of weapons to be found, you will find yourself playing for hours and hours getting everything in the game.
Like previously stated, the campaign is most fun when playing with others. However, like many other games that offer 4-player co op, things can get a tad hectic. When in a game with 4 players, it is easy to lose sight of your character (the only thing visually distinguishing yourself from your comrades is your color), and it might be a little harder to get some attacks in. I recommend only playing with one other friend, where you know what you're doing, what you're attacking, and especially where your character's at. 3 player co op is fun, but still kinda makes things confusing.
4 player is fun only when players are spread out (which is only about 20% of the time). When packed together, its basically impossible to know what's going on with everyone swinging swords.
Castle Crashers is a definate download to gamers who love action-adventure and co op games. It's worth 1200 points ($15), and it's easily worth the price due to the amount of time you'll put into it. I'd recommend it to just about anyone due to the 4 player co op and surprisingly fun multiplayer modes. It's an instant classic and the first truely fun and addicting XBL Arcade game.
NOTE 1: Currently, the online multiplayer (in both versus games and the Castle Crashers co op) is a bit... unstable. The issue is due to the amount of players playing the game online, and the developers just need to add more servers (at least that's based on my assumtions and my experience in online games. Hell, the same thing happened with Call of Duty 4). They will most likely add more servers, or add a patch that will somehow fix the problem.
NOTE 2: ALL modes (Castle Crashers and all Versus modes can be played on split-screen AS WELL as Xbox Live.