According to a Japanese Seiken Densetsu guide book (specifically, the one with the blue bottom and "SQUARE BRAND" in the bottom right corner), one of the game's programmers claimed that it apparently began life as an experimental tennis game.

"This game actually started out as an experimental tennis game. At some point, the court turned into scrollable screens, the racket turned into the playable character, the ball turned into a weapon, the opponent’s racket turned into enemies, and the “court” became referred to as the “map”… And before I realized it, a story was added into it and then the game was released as “Seiken Densetsu.” It was a curious experience."

The game started off as a project called "Seiken Densetsu: The Emergence of Excalibur" and was planned to be released in 1987 for the Famicom Disk System on 5 Disks, which would've been the biggest 8-bit game ever made. However resources were moved to focus on games for the Game Boy to which the project was canceled with Square giving refunds to all who had pre-ordered it. The Seiken Densetsu name was later reused for another game which was in development called Gemma Knights which would become Final Fantasy Adventure.

Final Fantasy Adventure, known as Seiken Densetsu: Final Fantasy Gaiden in Japan and Mystic Quest in Europe, is also the first game in the Mana series. The game would later be remade as Sword of Mana for the Game Boy Advance but with all Final Fantasy elements dropped.