Ilmmünster Abbey was formerly a collegiate foundation and originally a Benedictine monastery. The original site was a Benedictine monastery, founded in about 762 by the brothers Adalbert and Otker, traditionally supposed to be members of the Huosi, an ancient Bavarian noble family. The monastery was dedicated to Saints Arsatius, whose relics were here, and Quirinus, whose relics were at Tegernsee Abbey, founded by the same brothers, to which Ilmmünster was affiliated.
A collegiate foundation was also set up here in 1060. In 1493, the monastery was transferred to the Frauenstift in Munich, leaving the site for the Kollegiatstift. The property and assets were sold off in 1802 during secularisation, but the impressive church 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.