Rothenthurn ('red tower') castle may have existed since the 11th century. It was an estate of the Counts of Ortenburg and their successors, the Counts of Celje. A castle is documented in 1478, when the area was held by the Meinhardiner House of Gorizia. The present-day Renaissance building with its chapel was erected from the early 16th onwards, it was acquired by Christoph Khevenhüller about 1525 and afterwards in the possession of several local nobles over the centuries. Today the owners run the castle as a lodging establishment.
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.