At the end of the credits, the game states: "Watch out next year for Beyond Dark Castle," but this planned release was dropped due to the announcement of Return to Dark Castle, which is a sequel to Beyond Dark Castle in full color, including all the levels from Dark Castle and Beyond Dark Castle.
The game's credits announced a future release called "Beyond Dark Castle," but that plan was abandoned after the announcement of "Return to Dark Castle," a sequel that would be released in full color and include all levels from both games.