To be privately owned
Gollenfels Castle

Gollenfels Castle, a rock castle, is at approximately 287 meters above the Guldenbach valley in Rhineland-Palatinate. Situated opposite Stromburg Castle, it has a strategically advantageous position on the edge of the Soonwald forest.
History+
The origins of Gollenfels Castle are believed to be in the 10th or 11th century, possibly on the foundations of an ancient Roman fortress that secured a military road between Mainz and Trier. However, concrete evidence for this is lacking. Its first documented mention dates back to 1156, with another following in 1191. In 1450, Johann von Stein is named as an inhabitant.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://de.wikipedia.org/wiki/Datei:Gollenfels_main_building.jpg
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: 49.9446, 7.7686