On the level "Alien Ship", if the player goes left instead of right after being freed from their cell, they will see the game's producers in cryogenic stasis.

The clown princess Spoiler:at the end of the game is a reference to Tricky the Clown, an early animation made by one of the creators of the game.

Alien Hominid makes an appearance in Castle Crashers as an unlockable character. They also appear as enemies on the Alien Ship level.

Castle Crashers was first revealed at San Diego Comic-Con 2005 with the placeholder name "Ye Olde Sidescroller", and a demo running on the Nintendo GameCube. While IGN reported that the game was planned for release on the GameCube, PlayStation 2, and Xbox, 1Up.com reported there were no plans to release it on any sixth-generation platform at the time. It wasn't until the next year of the convention that the game received its official title and a shift to the Xbox Live Arcade was announced.