To be privately owned
Triptis Castle

The former lowland castle Triptis is situated in the town of the same name in Thuringia. Originally, it served to secure important trade routes and is now known for its keep.
History+
A local chronicle suggests the construction of the round tower as early as the 9th century, possibly in the context of conflicts between Germans and Slavs. Around 961, the first round towers are said to have been built in the region, similar to the keep of Osterburg in Weida. The tower, which was originally lower than it is today, was expanded into a castle complex, which also utilized the dammed town pond as a water obstacle.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://schloesserrundschau.de/thueringen/schloesser/saaleorla001.html
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: 50.7354, 11.8613