To be privately owned
Fortress Koblenz-Luisenturm

The Luisenturm, an octagonal tower, is a component of the former Prussian city fortifications of Ehrenbreitstein, now a district of Koblenz. It was named after Princess Luise.
History+
The construction of the Luisenturm was completed in 1856 according to designs by Karl August von Cohausen. Originally planned as an aesthetic element between Asterstein and the Pfaffendorfer Tor, the construction faced resistance from the Ministry of War. Through its connection with Princess Luise and the personal approval of King Friedrich Wilhelm IV, the project could be realized. The interior work, including a cast-iron spiral staircase, was completed in 1857. The tower was named in honor of Princess Luise and her engagement to Grand Duke Friedrich I in the same year. After the abandonment of the city fortifications in 1890, the city of Ehrenbreitstein acquired the tower, but only sold it in 1908 to Joseph Dornbach.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://de.wikipedia.org/wiki/Datei:Luisenturm_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: 50.3571, 7.6123