To be privately owned
Fortress Koblenz-Werk Klausenberg

The Fortress Koblenz-Werk Klausenberg, now known as the castle-like Villa Rheinburg, is a structure in Koblenz-Ehrenbreitstein. Originally built as part of the Prussian city fortifications, it now presents itself as a blend of historical defensive architecture and a private residence.
History+
Werk Klausenberg, also called Klausenbergturm, was constructed between 1827 and 1833. It was a caponier and thus an integral component of the Niederehrenbreitstein system of the Prussian Fortress Koblenz, which protected the city of Ehrenbreitstein. In 1850, adjustments were made to optimize the work for troop occupation. After the gaps in the fortifications were closed in 1854 and 1856, the caponier was fully integrated into the city's defenses.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://de.wikipedia.org/wiki/Datei:Rheinburg_Koblenz-Ehrenbreitstein_2012.jpg
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: 50.3595, 7.6148