Tunaborgen is a ruined former bishop castle. In 1291 Archbishop Nils Alleson mentioned a fortification on the site. The fort was also a strategic point in the Gustav Vasa's war against the Danes.
The ruins were rediscovered around 1920. The castle consisted of a square tower, a citadel, built together with an almost square walls and it was surrounded by a circular moat. Today the ground floor remains.
References:Sigmaringen Castle was first mentioned in the year 1077 in the chronicles of Petershausen monastery. The oldest parts of the castle are concealed beneath the alterations made during the 17th and the 19th centuries. The secret of the earliest settlement built on this defendable rock will never be fully revealed: large-scale excavation work would be necessary, which the extensive land development renders impossible. Judging from the many Roman remains unearthed in the area around Sigmaringen, the 12th century keep known as the 'Roman Tower' could be traced back to a Roman predecessor.
The castle remains that have been preserved (gate, great hall and keep) date back to the Staufer period around 1200. The castle remains were integrated into subsequent buildings. The foundations of the castle buildings are to a large extent identical to the surrounding castle wall.
These remains give us a good idea of how the castle might have looked during the 12th century.