To be privately owned
Hermannstein Castle (Ruin)

On the Schwarzenberg above the current Wetzlar district of Hermannstein in Hesse is the Hermannstein ruin, a former hilltop castle. It shows the remains of its residential tower, which is reminiscent of French keeps.
History+
The upper castle of the complex was built towards the end of the 14th century to secure the Hohe Straße and the Hessian border above the Lahn valley. The lower castle was added in the 15th century. Construction of Hermannstein Castle began after Count Johann IV of Solms-Burgsolms took advantage of unrest in Wetzlar in 1376 to seize control of the city. Landgrave Hermann II of Hesse then decided to build a counter-castle on Solms territory. By July 1377, building materials such as lime and stones were already being delivered, and although opponents tried to disrupt the construction, it could not be prevented.
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: 50.5797, 8.4941