Alte Burg, 'The old castle', in Burgsinn is a moated castle owned originally by the Barons of Thüngen. In 1337, the Bishop of Würzburg granted the castle and all its possessions as a fief to Dietz von Tungden, who largely built the present castle between 1339 and 1342.
The wide moat surrounding the castle is fed by an underground spring. On the west side, a bridge leads into the castle, which has a trapezoidal layout with towers at its corners. The outer wall is made of rusticated sandstone blocks, while the rest of the masonry is rubble stone, and the southern residential building features timber framing on its upper levels.
The square northeastern tower includes a small elevated residence. A pointed arch door leads into the southeastern tower, which was completed in 1543 by Martin I von Thüngen. The southern wall has been significantly altered with inserted windows, and the round-arched gate at the southwestern tower was fitted with corbels.
The castle complex is dominated by a square, five-story, 22-meter-high keep (dating to the 12th century), constructed of rusticated stones.
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.