Lengberg Castle, Medieval castle in Nikolsdorf, Austria

Lengberg Castle is a castle in Nikolsdorf, East Tyrol, about 13 km east of Lienz. Its core is a Romanesque two-storey palas with a 2.20-metre-thick ring wall and no bergfried. In 1480–1485 Virgil von Graben added a storey, a west wing with the chapel of Saints Sebastian and Nicholas, and an east wing; the castle gate was lowered by about 3 metres.

Lengberg Castle was first documented in 1190, when it belonged to the Counts of Lechsgemünde, a Swabian dynasty. In 1207 Count Heinrich sold it with Matrei and Mittersill to Archbishop Eberhard of Salzburg, and after Heinrich's death in 1212 it passed to the Archbishopric, which installed a burgrave and established the Lengberg court. In 1480 Virgil von Graben received the castle and rebuilt it in Gothic style.

The castle chapel was consecrated in October 1485 by Pietro Carlo, Bishop of Caorle, to Saints Sebastian and Nicholas. The ceremony and the festivities that followed were recorded by his secretary Paolo Santonino in his travel diary "Itinerarium". The chapel occupies the west wing added under Virgil von Graben.

The castle stands on a small hill on the northern side of the Drau valley in Nikolsdorf, about 13 km east of Lienz near the Carinthian border. It is now used by AufBauWerk, an organisation for young people, so the building is not open as a museum; view it from the outside and respect the residents' grounds.

A 2008 reconstruction uncovered a sealed vault on the second floor, likely filled during the 15th-century addition of the upper storey. Harald Stadler of the University of Innsbruck led the investigation of the debris, which held shirts, shoes and four linen bras. One closely resembles a modern bra with lace-making traces along its edges; curator Hilary Davidson of the Museum of London called it a missing link in the history of women's underwear.