To be privately owned
Kommern Castle

Kommern Castle, a historic hilltop castle, stands in the Kommern district of the city of Mechernich. This structure in North Rhine-Westphalia once served as an administrative seat and is now privately owned.
History+
The construction of Kommern Castle is dated to around 1350, with the residential tower likely originating from this early building phase. The castle was never used as a noble residence, but functioned until 1794 as the official seat of the Arenbergian bailiff for the Imperial Lordship of Kommern. The noble family of Arenberg, whose ancestral seat was in Aremberg, belonged to the high nobility in the 12th and 13th centuries and held the office of burgraves of Cologne. In the 17th century, they rose to dukes.
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: 50.6131, 6.6468