Burg Unterthal, Castle ruins in Thal, Austria

Burg Unterthal is a ruined hilltop castle in the hamlet of Linak, part of Thal, on a ridge between Kirchberg and Thalersee. Its core is a rectangular tower house built in the early 14th century, with walls up to 1.5 m thick, a barrel-vaulted cellar and a 16th-century west portal. A ring wall with keyhole embrasures from around 1500 and a 17th-century bastion with a sloped base survive.

Built in the 13th century by the Lords of Thal, a branch of the Lords of Graben, the castle was destroyed in 1292 during the nobles' revolt against Duke Albert I. The Windisch-Graetz rebuilt it in the 14th century and held it until Sebastian von Windisch-Graetz sold it in 1569 to Georg von Khevenhüller, who passed it to Archduke Karl III. In 1623 it went to Hans Ulrich von Eggenberg; his family let it decay, and a fire in 1715 ruined most habitable rooms.

The rectangular tower house, dating from the early 14th century, represents medieval defensive architecture with its stone barrel vault cellar.

The site is best reached on foot from nearby villages, as the hillside is accessible via local paths. There is minimal infrastructure on-site, so visitors should bring suitable footwear and allow time to explore the scattered remains.

The castle chapel, documented by 1322, served as a parish church into the 18th century and was demolished in 1772. The round tower, built in the 17th century, was converted into a summer apartment in 1996.