Castle Crashers is a beat 'em up video game developed and published by The Behemoth for the Xbox 360. It was originally released on August 27, 2008 by Microsoft Game Studios via Xbox Live Arcade as part of the Xbox Live Summer of Arcade. A PlayStation 3 version was released in North America on August 31, 2010, and November 3, 2010, in Europe via the PlayStation Network. A Microsoft Windows and OS X version, exclusive to Steam, was released on September 26, 2012. The game is set in a fictional medieval universe in which a dark wizard steals a mystical crystal and captures four princesses. Four knights are charged by the king to rescue the princesses, recover the crystal, and bring the wizard to justice. The game includes music created by members of Newgrounds.

On June 15, 2015, The Behemoth announced Castle Crashers Remastered for Xbox One, while the Steam version received it in the form of a free update. The remastered version features higher quality textures, higher frame-rate, performance improvements, and an additional mini-game. This version later released for the Nintendo Switch and PlayStation 4 on September 17, 2019. On August 6, 2025, the Painter Boss Paradise DLC was released for the Steam version of the game.

Castle Crashers is a side-scrolling beat 'em up that incorporates a small number of role-playing video game elements. After selecting a character, the player then selects a starting stage through an overworld map. After completing a stage, the player has the choice to revisit it or to move to another stage. The map also displays shops where the player character can buy items and weapons using coins gained from defeated foes, bosses, and chests. Arena stages can be unlocked where the player character can take on challenges to unlock additional characters such as the villager.

Castle Crashers supports cooperative gameplay for up to four players, either locally or online. The game progression in terms of what stages are unlocked is defined by the player who is furthest along, although some levels require all players to have unlocked them before proceeding to them; however, each player character will gain experience points and acquire wealth, weapons, and animal orbs independently as they progress with the rest of the party. In each stage, the player can use melee and combination attacks. Each character has a unique magical ability (e.g. Red Knight can create lightning bolts, Green Knight attacks with poison clouds, Blue Knight shoots out ice shards) and a health meter that, if drained from enemy attacks, will cause the character to fall in battle. In single player mode, this ends the stage; however, in cooperative multiplayer other players may attempt to revive the downed character.

Characters gain experience points by damaging foes which allow the character to level up. Level 99 is the highest level. Each level gained allows the player to allocate points towards the character's four basic combat attributes. Certain level advances also grant new combination attacks. Progress is tracked for each of the playable characters separately. The character'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. The player can find animal companions for their character that may assist in battle, improve the character's attributes, or provide another special ability such as fruits from defeated foes. The player can also pick up sandwiches in battle, which turn the player character into a muscular version of themselves, increasing attack power and opening certain doors. Beating the game unlocks Insane Mode, where enemies and bosses are ten times stronger and allows access to the Insane Store, where player can buy powerful but expensive weapons and animal orbs. The prices in the Insane Store were lowered in the remastered edition.

Each version of the game has two minigames. In Arena, the first minigame, player characters attempt to survive through several waves of enemies, or fight each other. This minigame is available on all versions. The Xbox 360, Mac OS X and Microsoft Windows versions feature a minigame called "All You Can Quaff," a button-mashing contest to attempt to eat as much food as possible. The PlayStation 3 version, however, features a Volleyball minigame for up to four players and four AI characters. The remastered version features a board game-like minigame called "Back Off Barbarian" where up to four players must avoid enemies and try to survive for as long as possible.

Castle Crashers is set in a fictional medieval universe. It begins with the character the player chose at the character selection menu attending a party in a king's castle. During the party, a dark wizard arrives, stealing a mystical gem and capturing four princesses. The king sends the non-player-character “grey knights” and the player to retrieve the gem, rescue his daughters, and defeat the wizard in battle. The knights encounter several enemies along the way, including other knights, multiple encounters with a cyclops and the conehead groom (ingame, coneheads are a group of people who wear dark grey armor, with the headpiece being very cone-shaped.), thieves, a giant "catfish", fencers, an industrial prince, ninja pirates, demons, a necromancer, a dragon, Medusa, a frost king, and aliens from Alien Hominid. As the player progress, they succeed in rescuing the princesses, and ultimately the journey culminates in a final showdown with the evil wizard. The player emerges victorious from the confrontation, having defeated the dark wizard, rescued all of the king's daughters, and recovered the mystical gem. The player then ride the reclaimed gem through several empty battlefields on their trip back to the castle. At the castle the king brings one of his daughters for one of the player(s) to kiss, her face is hidden throughout the entire game in the form of various visual gags. As with previous levels, the players fight to the death to claim a kiss from the princess. However, when the winner tries to do so this time, the princess is revealed to be a clown (A character that first appeared in an animation by Dan Paladin) that blankets the screen in the ending animation with penguins, weasels, kitties, hearts and rainbow beams.

Castle Crashers was first revealed on July 14, 2005, at San Diego Comic-Con under the working title Ye Olde Side-Scroller; however, the game did not receive its title until 2006 Comic-Con, when it was announced for Xbox Live Arcade. Though the original Comic-Con 2005 demo was shown running on a GameCube, no mention was made of a release on a Nintendo-based platform. It was released for the Xbox 360 on August 27, 2008. On July 23, 2009, The Behemoth announced that Castle Crashers would be coming to the PlayStation Network. The game was released on the PlayStation 3 in North America on August 31, 2010, and in Europe on November 3, 2010. A Microsoft Windows version exclusive to Steam was announced on August 16, 2012.