To be privately owned
Brestenberg Castle

Brestenberg Castle, a baroque gem in the Swiss Canton of Aargau, is not far from Lake Hallwil. Originally conceived as a country estate, its history ranged from noble owners to a sanatorium and a hotel, before returning to private hands.
History+
The origins of Brestenberg Castle date back to a 16th-century house. In 1622, Hans Rudolf von Hallwyl acquired the property and, in 1625, commissioned a new construction that shaped the baroque character of the present castle. In 1710, a farm building was added to the east side of the complex. After being owned by the von Hallwyl family and later the von May family, the castle changed hands several times.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://www.swisscastles.ch/Aargau/brestenberg.html
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: 47.3199, 8.2082