Part of an ancient complex emerged during work for the construction of an underground cistern within the hospital of “S. Maria del Popolo degli Incurabili”, on the via L. Armanni side, on the acropolis of the city. It comprised a large underground chamber, the walls faced with off cuts and small blocks of yellow tufa arranged in an irregular manner, with an _opus caementicium_ core; another wall in pseudo _opus reticulatum_; a niche and stretch of a conduit cut in a tunnel. The architectural typology, the facings of waterproof mortar and _opus signinum_, the cordons on the borders of the floors suggested that this was used as a cistern or _piscina limaria_. The _terminus ante quem_ was given by two middens built up against the cistern walls: the earliest fill dated to between the end of the 4th-beginning of the 5th century A.D., the other to around the end of the 6th century A.D. It may be suggested, on the basis of the account given by Lettieri to the Viceroy Don Pedro of Toledo, that these were structures connected with the first urban stretch of the Serino aqueduct which entered the city at Porta di S. Maria di Costantinopoli. It played a crucial role in the Graeco-Gothic war when, during a siege, it provided a way of entering into the city.