To be privately owned
The Castle can be visited
Johanniter Castle

In Kühndorf, Thuringia, the Johanniterburg is a historical structure built on the remains of an older complex. It is an example of a castle of the Order of St. John in Germany and later served as a residence for the Counts of Henneberg. Today, it is a privately run gem on the Thuringian Castle Road, open to visitors.
History+
The origins of the Johanniterburg date back to 1274, when a castle owned by the Counts of Henneberg was first mentioned. Remnants of this predecessor, such as the stump of a keep with bossed ashlar, can still be seen today. In 1315, this castle was demolished and replaced by a new construction by the Order of St. John. Unusual for the order in Central Europe, which had no military function there, a castellum consisting of two broad residential towers was built. After the death of high-ranking order members, the order lost interest in the castle, which was finally acquired in 1444 by Georg I of Henneberg-Römhild. He modernized the complex with a Zwinger system and five towers for defense with firearms.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: Mobile Geschichte
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: 50.6097, 10.4915