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:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.