Robotonia is a robot city, populated entirely by robots, that was founded by the Blue Knight. It strives to be a safe haven for robots everywhere and where they are allowed to "be free".
This episode in the 2003 anime series is named after the city, Robotonia, due to it being the topic of the episode.
It is located in the South Pole. The castle was then shown to be actually a spaceship, which went to space after the battle.