This was a two-storied “through and through” structure belonging to a later (probably 7th c. A.D.) construction phase. It was built in the open area between the central building complex and the Basilica.
The ground floor of the building was covered by a barrel vault and divided into four rooms by transverse arched openings. Two narrow stone staircases (width 0.50 m.) still preserved on the facades of the building led to the upper floor, which operated as a cistern that also collected rainwater from the roofs of the Basilica and east corridor. Its floor was covered by four rows of bricks, and its interior walls were coated with hydraulic crushed-tile mortar to protect it from moisture.
Surplus water from the cistern was channeled through clay pipes into the cistern situated north of the baths.