Where: Schönbühel-Aggsbach, Austria

Start of construction: around 1172

End of construction: 19th Century

Accessible: no, private property

Important fact:

Built on a 40 meter high rock over the Danube River. Privately owned since 1929
by the counts of Seilern-Aspang.

https://www.schoenbuehel.at/Schloss_Schoenbuehel