Mr. Do’s Castle is a platform game released in arcades by Universal in September 1983 and later ported to various platforms, including the Atari 2600. In the game, the player controls Mr. Do, who must score points by collecting cherries and/or defeating unicorn-like monsters within a castle setting filled with platforms and ladders. The player can use a hammer to knock out blocks containing cherries, while empty holes left by the knocked-out blocks serve as traps for the monsters.
To defeat monsters, the player can cause a block above the trapped monster to fall on top of it. If the player takes too long to complete a level, the monsters transform into faster, more difficult forms. The game progresses to the next level when all cherries are collected or all enemies are defeated. The player loses a life if Mr. Do is caught by a monster, and the game ends when the player runs out of lives.
Similar to the original Mr. Do!, the player can earn an extra life by collecting all of the letters from the word “EXTRA.” This can be achieved by collecting all three keys distributed around the playfield and then picking up a magic shield from the top floor. Monsters can be transformed into more easily defeated forms bearing the EXTRA letters. After a brief interval, they change back into their normal forms. Additionally, the game offers a bonus credit for collecting a rare diamond that appears on the playfield at random intervals.
Some versions of the game, such as Mr. Do Vs. Unicorns and early revisions of Mr. Do’s Castle, do not feature cherry blocks. Instead, fill blocks change colors every two stages, and there are only three ways to complete a level in these versions. This adds an extra layer of challenge and variety to the gameplay.