To be privately owned
Efferen Castle

In Hürth-Efferen, Efferen Castle is a structure in the vicinity of Cologne. Its medieval gate tower is considered the oldest building in the city of Hürth, a moated castle once fed by the Duffesbach stream.
History+
The exact period of the first castle's construction remains unknown. Originally, the convent of St. Maria im Kapitol, founded by Queen Plektrudis around 696, was the lord of the manor in Efferen. The Counts of Jülich served as bailiffs and enfeoffed Cologne nobles with the estate. Towards the end of the 14th century, the Cologne Overstolzen family emerged as the first lords of the castle, later naming themselves after the castle. Between 1391 and 1393, the castle was fortified by the people of Cologne and Jülich as an outpost against the claims of Archbishop Friedrich III of Saarwerden of Cologne, with the gate tower likely originating from this period.
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: 50.8950, 6.8963