Castle Crashers is well deserving of all the good reviews it gets. Not only is it a great Action/Adventure game, you sort of get an RPG element in there with the leveling up and putting points towards stats. Just because it has a couple of RPG elements in it does not make it an RPG by any means, let's make that clear.
A key feature of this game is the 4 player Co-Op gameplay. This can be done online or locally. There are plenty of characters to choose from once you unlock them or buy DLC which includes characters. Weapons become available while progressing through the story as well, some of which are useless while others are unusable for quite a few levels. You can even collect little animals that each have special powers to aid you in your quest.
I do have a gripe or two with this game. One is that you have to be very exact when you attack enemies, one or two pixels up or down from the enemy will cause you to miss them. While you miss them easily they have super aim and when the enemy shoots arrows look out because they are extremely accurate. Another gripe is that it takes such a long time to level up that you will be bored of the game before you can get to the max level.
All in all, this XBLA game is one of the best games to come out on Xbox 360 so far. Given it's nearly endless amount of replayability and fun, it's worth the 1200 MSP. As always though it never hurts if you can get this game for cheaper!