The Castle can be visited
Pfaueninsel Castle

Pfaueninsel Palace, a pleasure palace, stands on the island of the same name in the Havel. Originally conceived as a retreat for King Frederick William II, it has a romantic design as an apparent ruin.
History+
The creation of Pfaueninsel Palace is linked to King Frederick William II, who had a summer palace built on Heiliger See in Potsdam at the end of the 18th century. From there, a visual axis was created to what was then Kaninchenwerder, today's Pfaueninsel. The king desired a small palace at this location, to serve as a resting place after boat trips and to allow for overnight stays. The planning of the building, constructed between 1794 and 1797 under the direction of master carpenter Johann Gottlieb Brendel, envisioned ruin architecture, to be made exclusively from Prussian woods. Wilhelmine Encke, the long-time confidante and later Countess Lichtenau, was involved in the design of the interiors. King Frederick William II died in the year of its completion, and she was sent into exile.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: Joachim Schneeleopard
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: 52.4309, 13.1199