To be privately owned
Mittelburg Castle

Mittelburg Castle in Neckarsteinach, located in the Bergstraße district of Hesse, is a well-preserved medieval hilltop castle. It is the second oldest of the four castles in the region.
History+
The construction of Mittelburg Castle is presumed to have taken place around 1165, initiated by Conrad I von Steinach. Originally, the castle was divided: half was a fief of the Bishopric of Worms, and the other half was allodial property. After the death of Boppo von Steinach in 1325, Conrad's last descendant, the Worms half was inherited by Luzzo von Helmstatt, while the allodial part went to Conrad zu Erbach, who promptly sold it to Archbishop Matthias von Mainz. The Landschad von Steinach family was able to reacquire full possession of Mittelburg Castle in the 16th century.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://www.wehrbauten.de/thread.php?threadid=984
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: 49.4095, 8.8327