Mr. Do's Castle is an action-platform game for the Atari 2600 in which the player controls Mr. Do, navigating single-screen levels to collect cherries and defeat unicorn-like monsters while avoiding obstacles. The gameplay emphasizes timing and strategic use of ladders and blocks to trap enemies, offering a blend of arcade-style scoring and puzzle elements. Notable for its faithful adaptation of the 1983 arcade original and its distinctive fantasy castle setting.