Castle Heimenburg, Hainburg an der Donau, Medieval castle ruins on Schlossberg Hill in Hainburg an der Donau, Austria.

Castle Heimenburg is a ruined hilltop fortress above the town of Hainburg an der Donau, in Austria. Standing stone walls mark the outline of the old structure, and the elevated position gives a clear view over the Danube and the flat land on both sides of the river.

A castle stood at Bad Deutsch-Altenburg until Henry III destroyed it in 1042; around 1050 he ordered a new Heimenburg built on the Schlossberg above Hainburg. In 1252 Margaret of Babenberg married Ottokar II Přemysl in the castle chapel, and Ottokar enlarged the residential tower in 1260. The castle withstood three sieges by Gábor Bethlen in 1619/20, but was stormed in 1683 during the second Turkish siege, when 8,432 townspeople and refugees were reportedly killed. After a new palace was built at the foot of the hill in 1742, the fortress fell into decay.

The name Heimenburg reflects the castle's role as a protective stronghold watching over the town below. Visitors standing among the remaining walls can still trace the outline of the old courtyard and get a sense of how the space was once organized.

The ruins are reached on foot via marked trails from Hainburg an der Donau, and the walk uphill takes a short time. Sturdy footwear is a good idea, as parts of the path can be uneven.

In 1683, during the second Turkish siege, the castle was stormed and, according to tradition, 8,432 townspeople and refugees who had taken shelter there were killed. The site was already partly ruined before 1742, when a new palace at the foot of the hill made the old fortress useless.