The object of "Mr. Do's Castle" is to score as many points as possible by collecting cherries and defeating unicorn-like monsters. The game takes place in a castle filled with platforms and ladders, some of which can be flipped from one platform to another. The player controls Mr. Do as he collects cherries by using a hammer to knock out blocks that contain them from the various platforms. Empty holes left by the knocked-out blocks serve as traps for the monsters - if a monster falls into a hole, the player can then defeat it by causing a block above the 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 that occasionally multiply. The game advances to the next level when all cherries on the level have been collected or all enemies have been 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.