The Radaxian Castle is the home of King High Stone, ruler of the Kingdom of Niban. It is located to the east of Mt. Eternal and to the west of Cragg Lake. It is the sixteenth level of Alex Kidd in Miracle World.
Castle Niban is inhabited by the Nibanian royal family, including High Stone. Alex ventured to the castle after he saved his brother Igul. He gave him a letter which Alex gave to High Stone in exchange for the Hirotta Stone, which he eventually uses to defeat Janken the Great.