Milon's Secret Castle is an action-adventure game developed and published by Hudson Soft. It first launched in Japan on November 13, 1986, and later released in North America in September 1988. The game follows Milon, a boy from a music-focused planet who cannot understand music. When the evil warlord Mahler attacks his village Ishtar, steals the villagers' instruments, seizes Castle Garland, and imprisons Princess Eliza, Milon sets out to infiltrate the castle to rescue the princess and recover the stolen instruments. Players control Milon to run, jump, and use magic bubbles to defeat enemies and break bricks, searching for hidden items and passages. Filled with enemies and hidden power-ups, the castle requires players to solve puzzles and fight through each floor, eventually facing Mahler to complete the rescue mission.