Where: Berlin, Germany

Constructed: 1794-1797

Accessible: yes, interior with guide

Important fact:

Built as a gift from Friedrich Wilhelm II von Preussen to his mistress. The castle is located on a 98-acre island, which can only be reached by ferry. The castle survived all wars without damage.

https://www.spsg.de/en/palaces-gardens/object/peacock-island-house/