To be privately owned
Bartelstein Castle

Bartelstein Castle, also known as Bartelstein Fortress, stands on a limestone rock on the left bank of the Danube near Scheer. This structure in the Sigmaringen district, Baden-Württemberg, is a testament to bygone eras.
History+
The origins of the estate date back to the 11th and 12th centuries, when the first Bartelstein Castle was built. Knight Ortolf von Bartelstein is considered the first known nobleman to reside here. The original complex was located on a rock next to the current castle, above a system of limestone caves, one of which provided access to the fortress.
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: 48.0754, 9.2950