To be privately owned
Kohlstädt Castle (Ruin)

In the southern Teutoburg Forest lies the Kohlstädt Ruin, also known as Kleine Herrenburg. The Frankish-Norman complex served as a customs and supply station.
History+
Kohlstädt Castle was constructed around 1100 AD, with the Frankish-Norman style being dominant. It was likely built by the Counts of Schwalenberg, the protectors of Corvey Monastery. In 1365, the complex passed into the hands of the Lords of Lippe and served them as protection towards Paderborn. By the end of the 14th century, the castle was abandoned and left to decay. There were various interpretations of its use, from an iron smelter to mythical attributions. Today, it is known as a former customs and supply station that secured an important pass and a crossing point on the Sennehellweg.
Source: Wikipedia · Licence: CC BY-SA 4.0
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 51.8250, 8.8606