The remains of the Castle can be visited
Merkenstein Castle (Ruin)

The castle ruin of Merkenstein in Lower Austria near Bad Vöslau is the remains of a fortress. Its history dates back to the 12th century.
History+
The earliest uncertain references to Merkenstein Castle date back before 1141. A confirmed mention is found around 1170 in the Codex Falkensteinensis. In 1486, the castle was conquered by King Matthias Corvinus of Hungary. Subsequently, it was administered by imperial stewards before being owned by the Heißperger family from 1603 to 1672, and then passing to the Dietrichstein family. In 1683, Ottoman troops captured and destroyed the castle. In the 16th century, the Merkenstein dominion included Pottenstein, in addition to Gainfarn and Großau, as well as the administrative districts of Furth, Muggendorf, and St. Veit.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://en.wikipedia.org/wiki/File:Burg_Merkenstein_06.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: 47.9823, 16.1337