To be privately owned
Fort X (Köln)

Fort X in Cologne, also known as Prince Wilhelm of Prussia, is a fortification located in the Neustadt-Nord district.
History+
The construction of Fort X took place between 1819 and 1825. The design originated from the Oberbaudeputation in Berlin, while the execution was carried out by the master builders Jaschnik, Elsner, and Buschius. By the late 1880s, the fort was integrated into Cologne's inner rampart system. In 1912, the first apartments were created within the complex. Fritz Encke developed a plan for a green fort in 1914, and its garden facilities, including a rose garden on the rampart, were realized in 1919/20. In the 1930s, the fort's reduit housed the office of the municipal garden department and several tenants, including a gardener.
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: 50.9575, 6.9617