The Seven Halls, or Sette Sale, is the name of the complex of cisterns located on the Oppian Hill. Previously believed to be connected to Nero's Domus Aurea, they were later found to be a large cistern supplying the Baths of Trajan. The cisterns were fed by a branch of the Trajanic Aqueduct. Found beneath the complex were the remains of a grotto lined with slabs of marble belonging to the Domus Aurea. In the fourth century CE, a domus was built on top of the complex, likely from a building used to service the tank itself. In the Middle Ages, the northernmost chambers were used as catacombs.
The complex, still quite well preserved, is built into the side of the Oppian Hill, shaped to conform to the terrain. The cisterns comprise nine (not seven) parallel chambers. The name Seven Halls comes from the fact that, when the complex was noted in the mid-18th century, only seven chambers were recognized.
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.