To be privately owned
Marienburg Castle

Marienburg in Monheim am Rhein is a brick building, originally conceived as a country house. It is a remarkable example of Cologne Neo-Gothic architecture, distinguished by its design and the accompanying English-style garden park.
History+
The Marienburg was constructed between 1879 and 1880, based on designs by the Cologne architect August Carl Lange. The patron was Eugen von Kesseler, a Cologne district court judge and member of the Reichstag, who used the estate as a summer residence. With the name 'Marienburg,' von Kesseler aimed to establish a connection to the Teutonic Order castle in West Prussia.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://www.wehrbauten.de/thread.php?threadid=583
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: 51.0877, 6.8833