Hardenstein Castle (German: Burg Hardenstein) is a ruined castle in North Rhine-Westphalia, Germany. The remains lie east of Herbede on the Ruhr River, surrounded by mountains, and are not easily accessible. Nearby ruins show that the castle was once part of an important mining centre, probably dating to the Middle Ages; the earliest records, from the 16th century, support this. The castle features in the legend of the Nibelungs. The castle's association with mining led to a legend that King Goldemar, a dwarf or kobold, dwelled there.