The Osterburg castle ruins are the ruins of a hilltop castle in the cadastral community of the same name in the municipality of Haunoldstein. The ruins stand on a triangular rock plateau that slopes steeply southwards into the Pielach Gorge.
The first structure was probably built in the last quarter of the 12th century, the builders are unknown. Around 1200, the Counts of Peilstein owned the castle as freehold. Count Friedrich V of Peilstein died here in 1209. In the second half of the 13th century, the castle was inhabited by followers of the Häusler family. They named themselves after the Osterburg, which had meanwhile become a sovereign fief. Through marriage, it passed from the Häuslers to Konrad Eisenbeutel at the beginning of the 14th century. The Tursen of Tiernstein inherited the property around the middle of the 14th century. According to an old inscription above the door of the palace, Rudolf Turso von Tiernstein had the castle expanded in 1405. In 1489 the Prüschenk brothers took over the rule. In 1514 Emperor Maximilian I enfeoffed the knight Hans Geyer, whose family had only moved here from Franconia in 1482, with the castle. The castle was modernized and in 1584 the Geyer family acquired a district court from the sovereign. The Protestant branches of the Geyer family had to leave the country after the Battle of the White Mountain. The line that remained Catholic was elevated to the rank of baron in 1650 and was allowed to call itself Geyer von Geyersperg at Osterburg. However, the line was soon heavily in debt and was sold just two years later to Georg Wertemann Freiherr von Wertema. He passed it on to Horatius Buccellini Freiherr von Reichenberg in 1653. Finally, Count Raimund Montecuccoli, who owned the neighboring Hohenegg Castle, bought the property in 1668. From 1675 onwards he owned it as freehold. The administration was concentrated in Hohenegg and later in Mitterau, and as a result Osterburg was abandoned and fell into disrepair. In 1766 Count Zeno Montecuccoli had the medieval fortifications and some of the residential buildings demolished. The material was used to expand Mitterau Castle. The remaining ruins belonged to the Montecuccoli family until 1983. After that it became private property, and from 1985 onwards it was made habitable again and partially renovated.