Castle Crashers is a side-scrolling, beat-em-up which incorporates a small number of role-playing game elements. - After selecting a character, the player then selects a starting level (beginning with the wizard's attack on the castle) through a graph-like world map; once the player completes a level, they may then opt to start or revisit that level any time. Other levels include shops where the player can buy items for healing, attacking, or defense using coins gained from defeated foes, and arena levels where the player can take on challenges to unlock additional characters, although in co-op the arena turns into a player vs player match. This goes for the same as when the players defeat the boss, freeing the princess, and if there are two or more, the battle starts to see who gets the princess' kiss. - In each level, the player can use a number of melee attacks and combination attacks in addition to a unique magical ability each character has in order to defeat foes. The player has a health meter that, if drained from enemy attacks, will cause the character to fall in battle; if the player is playing alone, this ends the level and the player will have to start it over; otherwise, any of the other players may attempt to revive the downed character. The player's magic level is also tracked by a meter and regenerates over time. Numerous weapons can be found in the game, each that have various effects to the character's attributes when equipped. There are also animal orbs that can be found that may help out in battle, improve the character's attributes, or provide another special ability such as gaining more treasure from defeated foes. After a player damages an enemy, they gain experience points, eventually gaining the player a new level. Each level gained allows the player to allocate points towards four basic attributes to improve that character in combat. One character can "level up" to level 99 which is accomplished by amassing exactly 113,680 exp points. Certain level advances also grant new combination attacks. Progress in experience and game completion is tracked for each of the playable characters separately. New playable characters can be earned by completing the game with a new character, or through downloadable characters from Xbox Live Marketplace, such as the titular character from Alien Hominid.