Karlstein Castle, locally known as Burg Karlstein, lies in the village of Karlstein an der Thaya, east of the city of Zwettl in the province of Lower Austria in Austria.
Karlstein Castle was built on a high cliff above the Thaya river and was first mentioned in the 12th century. Between 1390 and 1470 it was owned by the Truchessen von Grueb family. They were followed by the Hauser family and later the Puchheim and Van der Straaten family.
Because of its location close to the border between Bohemia and Austria, Karlstein Castle and the surrounding area were the scene of a lot of quarrels with the Bohemian King Ottokar, the Hussites and the Hungarian King Mathias Corvinus.
After the Farmer Rebellion of 1597 was crushed, its leader Andreas Schrembser was probably incarcerated in Karlstein Castle before he was quartered in Waidhofen an der Thaya. In 1645, during the 30-Years War, the castle was sieged by Swedish troops but was not taken.
In the 18th century a firm producing watches was located in the castle. In 1832 the castle was abandoned and slowly fell into decay.
From 1914 until 1918 the empty and dilapidated Karlstein Castle was used as an internment camp for citizens from hostile countries for Austria and Hungary. After World War I the Hungarian revolutionary Bela Kun was also interned here.
At present the castle is used for cultural activities. A nice castle in a sleepy little village.