To be privately owned
Arnstein Castle (Eichenberg)

In the Eichenberg district in the North Hessian Werra-Meissner district is Arnstein Castle, a manor house that emerged from a late Gothic castle and has been redesigned several times. It lies on a narrow ridge and is now privately owned.
History+
The origins of the Arnstein castle complex date back to the 12th century, when it was built as a border castle against Braunschweig and the Mainz Eichsfeld. In 1264, at the end of the Thuringian-Hessian War of Succession, the castle was awarded to Landgrave Heinrich I of Hesse. Later, in 1337, the Lords of Rusteberg were mentioned as pledge holders, but they acted as robber knights. This led to a joint action by the Archbishop of Mainz and the Landgrave of Hesse in 1342, who expelled the Rustebergers and installed the Lords of Berlepsch as new pledge holders. The castle was destroyed in 1396 but rebuilt.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: Jan Stubenitzky (Dehio) / CC BY-SA 3.0 via Wikimedia Commons
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: 51.3601, 9.9075