The Monastery of St. Michael on the Heiligenberg (Saints" Mountain), was a branch of the nearby Lorsch Abbey. The ruined complex that can be seen today was built beginning in 1023. Within the nave are traces of the Roman temple of Mercury. The monastery was abandoned in the 16th century.
The first mention of the monastery is in the Lorsch codex, from the 12th century, which dates the founding of the monastery to 870. No remains of an earlier monastery have been found, so this might have been merely an estimate of the founding.
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.