To be privately owned
Ahorn Castle

In the district of Coburg, Ahorn Castle has a history dating back to the 11th century. Originally a castle complex, it has been redesigned several times and today presents itself as a Renaissance castle with round corner towers and a staircase tower in the inner courtyard.
History+
As early as the late 11th century, a "Burg Ahorny" was mentioned in donation documents. It was part of the inheritance of the Polish Queen Richeza, who bequeathed her property around Coburg to Archbishop Anno II of Cologne. He used it to endow the Benedictine monastery of Saalfeld, which he founded in 1071.
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: 50.2364, 10.9488