To be privately owned
Mülheim Castle

A historic castle, the Teutonic Order Commandery Mülheim, is located in Warstein-Mülheim in North Rhine-Westphalia. Originally founded by the Teutonic Order, it shaped the region for centuries.
History+
The Teutonic Order Commandery Mülheim was founded between 1266 and 1268, when a knight from Mulnheim bequeathed his property to the Teutonic Order. After initial difficulties in acquiring the patronage rights of St. Margaretha's parish church, which eventually passed to the order in 1275, the establishment was elevated to a convent in 1290. Despite an initial expansion of its possessions, the commandery fell into crisis in the late Middle Ages, leading to the dissolution of the original convent. Nevertheless, it retained its importance as a training center for knights, particularly for missions in Livonia.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://www.zeche.org/
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: 51.4902, 8.2791