To be privately owned
Wildegg Castle

Wildegg Castle, a historic building situated in the Vienna Woods near Sittendorf, now serves as a youth camp. Its uniqueness lies in its triangular Renaissance courtyard, considered the first north of the Alps.
History+
The construction of Wildegg Castle is presumed to have taken place between 1136 and 1188, with Heinrich von Wildegg considered the probable builder. Its first documented mention occurred in 1188 under the name “Wildekk.” As part of a chain of castles on the edge of the Vienna Woods, it served to protect against incursions from the east. Originally conceived as a rock castle, it was later expanded with a tower and a chapel. The castle remained in the possession of the Wildegg family until 1261, before passing to the Altenburger family through inheritance. After 1346, ownership changed frequently until the Neideckers acquired the castle and converted it into a Renaissance palace. Under the Neideckers, who embraced the Reformation, conflicts arose with Heiligenkreuz Abbey.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://www.wehrbauten.at/noe/niederoesterreich.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: 48.0861, 16.1652