To be privately owned
Hornstein Castle

On a wooded slope north of Krumpendorf am Wörthersee in Carinthia, Austria, lies Hornstein Castle. The structure has a cubic shape and round corner towers.
History+
The origins of Hornstein Castle date back to the 15th century, when Ulrich Hornsteiner commissioned its construction. The castle changed hands several times. In the mid-18th century, it was owned by the Counts of Ursenböck, who received a license for mass in the castle chapel in 1757. In 1865, Otto Daublesky Freiherr von Sterneck acquired the property. Markus Pernhart captured the castle in the mid-19th century as part of his castle cycle.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Hornstein_06.jpg?uselang=de
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: 46.6362, 14.2258