To be privately owned
Röttgen Castle (Köln)

The castle was built as a knight's seat from the 9th to the 13th century and demolished in 1790. It was not rebuilt in the Romantic style until 1866. The associated area was enclosed with a wall. In 1909, the Eau de Cologne manufacturer Peter Mülhens bought Röttgen Castle. There, in 1924, he founded the Röttgen Thoroughbred Stud. During the Second World War, the castle was severely damaged by bombs. From 1945 to 1953, it served as the seat of the British military administration. In its rooms, Chancellor Konrad Adenauer negotiated the Germany Treaty with the victorious powers.
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: 50.9136, 7.0816