The Castle can be visited
Belvedere Castle

Belvedere Palace, a twin-towered complex, is in Potsdam. It is an example of Italian Renaissance architecture in Brandenburg, with expansive views over the surrounding landscape.
History+
As early as 1793, under Frederick William II, there were plans for a Belvedere on what was then Judenberg, but these failed due to high land prices. Decades later, under Frederick William IV, the “Romantic on the Throne,” the idea was revived. The site, by then renamed Pfingstberg and royal property since 1817, inspired the artistically gifted king to create his own sketches. He drew inspiration from the Casino of Villa Caprarola in Italy. The construction, based on the king's designs, was entrusted to architects Ludwig Persius, Friedrich August Stüler, and Ludwig Ferdinand Hesse, while Peter Joseph Lenné designed the surrounding gardens. The first construction phase lasted from 1847 to 1852. Financial constraints led to an interruption in favor of the Orangery Palace.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://de.wikipedia.org/wiki/Datei:Belvedere_auf_dem_Pfingstberg.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: 52.4188, 13.0589