BS Do-Re-Mi

The player controls Milon by running, jumping and shooting bubbles. The player starts out at the bottom floor of a four-story castle, named Castle Garland, and must navigate upward, searching the three main stories, the well, two side towers and the very top. If the player remains outside of the castle for too long, lightning bolts start falling from the sky. Each room of the castle contains many enemies and hidden items.

Milon's bubbles are both a weapon and a tool; throwing bubbles at soft blocks causes them to break apart and either create additional routes, reveal hidden shops, or expose specific items. The bubbles can also destroy the demons that inhabit the given rooms, though they will come back to life after several seconds. Each story can only be reached by defeating the first boss. Although seemingly basic, the only way the bosses can be reached is by finding a host of secrets, some of which require sheer luck or a helpful manual.

The protagonist of the game is Milon, a boy who suffers from tone deafness. Milon lived in Hudson land, where its denizens communicate with each other using tone. Milon then decides to travel throughout the land of Hudson. Before leaving for his trip, Milon visits Queen Eliza who lives in Castle Garland.

However, when Milon arrives at castle Garland, the castle is already invaded by an evil wizard named Maharito (Warlord of the northern region in the English game manual) and his demonic army. They are terrorizing Hudson, seizing the people's instruments of music, and occupying the castle. In response to this situation, Milon decides to enter the castle to fight the invaders, rescue the queen, and return the musical instruments seized by the forces of Maharito from the people of Hudson.

Upon release in 1986, Famicom Tsūshin (Famitsu) magazine gave the original Famicom version a score of 29 out of 40. The Wii Virtual Console version released in 2007 later received a negative reception. GameSpot's reviewer Frank Provo described the game as one of the worst games, as he criticized the control scheme, giving it a "terrible" and score of 2.0/10. Power Unlimited gave the Game Boy version a score of 83%; they noted the game lacking originality, but they praised its gameplay and graphics.

In 1996, a Super Famicom sequel to the game was released only in Japan, known as DoReMi Fantasy: Milon no Dokidoki Daibouken.

There are some references to Milon's Secret Castle in a few Hudson Soft games. Milon made a cameo appearance in the Sega Saturn game Saturn Bomberman, and in Star Soldier there are hidden Milon tiles that can only be uncovered and destroyed for a 40,000-point bonus when certain score conditions are met.