Bunkenburg

Bunkenburg was a castle built during the 13th and 14th centuries in the shape of a circular fort located on the banks of the Aller opposite Ahlden in north Germany. Only a section of the rampart, roughly 60 metres long and 3 metres high, exists today. The name of the castle is probably derived from the material, bog iron, used for its construction and known in common parlance as Bunke.

Source: Wikipedia