To be privately owned
Kyllburg Castle

Kyllburg, a historic castle complex, is in the town of the same name in the Bitburg-Prüm district, Rhineland-Palatinate. Once built to protect the Archbishopric of Trier, it stands today.
History+
Construction of Kyllburg began in 1239 under Theoderich, Archbishop of Trier. Its purpose was to safeguard the Archbishopric of Trier against the lords of Malberg. To finance the construction, Archbishop Theoderich, with the consent of the cathedral chapter, sold lands to St. Thomas Abbey. Shortly thereafter, the castle was incorporated into the town's fortifications. Theoderich's successor, Arnold II, had the castle and adjacent houses surrounded by a wall in 1256 and concluded an agreement with the Abbot of Prüm regarding their respective fortifications. The original, over 23-meter-high keep remains preserved to this day.
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: 50.0380, 6.5879