To be privately owned
Marsbach Castle

Marsbach Castle, a historic edifice with the character of a fortress, is situated in the Danube Valley near Hofkirchen im Mühlkreis in Upper Austria. As the oldest aristocratic seat in the upper Mühlviertel region, it offers a view over the Danube, which was once of strategic importance for the feuding knights residing here.
History+
The first documented mention of Marsbach Castle occurred in 1075, when it was referred to as “castrum morspah” and Raffoldus de morspah. The fortress was the private property of the Diocese of Passau, and the Marsbach family served as episcopal ministerials. In the 13th century, there were conflicts over the castle. Heinrich von Marsbach was forced to surrender the castle to the Bishop of Passau, and Otto von Marsbach later seized the complex to make trade routes unsafe. It was not until 1288 that the dispute was settled by Duke Albrecht I of Austria and Duke Heinrich of Bavaria, with the Marsbach family being compensated with possessions in Bavaria, and the castle returning under Passau rule. The Marsbach family died out in 1380.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Burg_Marsbach_-_Austria_-01.jpg
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: 48.4581, 13.8231