Schloss Freyenthurn, Renaissance castle in Klagenfurt, Austria.

Schloss Freyenthurn is a Renaissance castle in Klagenfurt perched on the southern slope of Falkenberg hill, rising above the surrounding landscape with towers and prominent bay windows. The building combines original Renaissance features with Victorian modifications added in the 19th century.

King Ferdinand I's treasurer, Hans Angerer, built the castle in 1541 and received permission to add 'zu Freyenthurn' to his name upon completion. In 1884, major renovations integrated Tudor architectural elements, giving the building its present character.

The name Freyenthurn, 'free tower', was adopted by builder Hans Angerer, who from 1541 called himself Hans Angerer zu Freyenthurn. The estate passed through many noble families, including the Attems, Grotta von Grottenegg and Koller. In the 20th century it housed a Schlosshotel and later a planned sanatorium; since 2003 it has been leased to the operator of a brothel.

The castle stands on the southern slope of the Falkenberg above the eastern bay of Lake Wörthersee, with the city vineyard directly below it to the west. It is a heritage-protected building and not open as a museum; since 2003 a Vienna investment company has owned it and leased it to the operator of a brothel. View it from the outside, from the vineyard or the lakeside road, rather than expecting entry.

The castle's name gave its builder a new identity: after completing it around 1541, court paymaster Hans Angerer was allowed to call himself "Hans Angerer zu Freyenthurn". The building later passed through many hands, and since the early 2000s it has been leased to the operator of a high-end brothel.