To be privately owned
Hubertus Castle

On the Rosenberg hill in Graz's Geidorf district stands the Hubertus-Schlössl, a historic manor house. It was built in the late 16th century and its architecture has been expanded and refined.
History+
The origins of the Hubertus-Schlössl date back to 1590, when construction began on the slopes of Graz's Rosenberg. Additions were made in the last quarter of the 19th century and again in 1904, expanding the original building. Notable features include the wrought-iron gate wings, dated 1778, and another wrought-iron grille in the staircase from the third quarter of the 18th century.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://de.wikipedia.org/wiki/Datei:Hubertus-Schl%C3%B6ssl_Graz.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: 47.0887, 15.4475