To be privately owned
Hardegg Castle

Hardegg Castle Ruin, a Romanesque twin castle, stands on a wooded hill near Zweikirchen in Carinthia, Austria. The remains of this complex are now a protected monument.
History+
Hardegg was first documented in 1134 with the mention of Mengotus de Hardeche and his son Gotpoldus. However, the Hardegg family, who served as ministerials to the Carinthian dukes, died out as early as 1176 with Hertwig von Hardegg. Later, in 1264, Seifried von Mahrenberg redeemed the castle as a donation to the Bishop of Bamberg for the construction of a Cistercian monastery. Since Seifried died shortly thereafter, the monastery construction did not materialize, but Hardegg initially remained in Bamberg's possession before returning to the Carinthian dukes in the early 14th century. Duke Albrecht of Austria enfeoffed the castle to Friedrich and Konrad von Auffenstein in 1346. They were followed by the Khevenhüller and, in the early 16th century, the Leininger brothers as lords of the castle. In 1527, Hardegg was granted as a fief to the Carinthian mint master Hieronymus Puecher. From the 17th century onwards, the castle began to decay, a process accelerated by rapid changes in ownership and possibly also by the introduction of the roof tax in the 18th century.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://www.wehrbauten.at/ktn/kaernten.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: 46.7172, 14.2581