To be privately owned
Mammertshofen Castle (Roggwil)

Mammertshofen Castle in Roggwil, Canton Thurgau, has one of Switzerland's most formidable keeps. This complex was once a noble residence.
History+
The origins of Mammertshofen Castle date back to the 13th century, when it served as the seat of the Marshals of Mammertshofen, whose lineage died out around 1360. Later, the castle came into the possession of the Schenken von Castell. During the Appenzell Wars, the complex was taken without a fight and thus remained undamaged. It remained with this family until 1645. In 1792, the Orlandi family from Bevers acquired the estate for 80,000 guilders, which included approximately 108 hectares of land, forest, four farms, and two vineyards. Since this purchase, the castle has continuously remained in the private ownership of this family. The 13th-century keep contains boulders weighing up to three tons in its masonry.
Source: Wikipedia · Licence: CC BY-SA 4.0
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: 47.4946, 9.3981