To the south-east of Trajan's basin is an internal wall (opus latericium) with three square towers, connecting the basin and the city wall to the south-east (22). The maximum preserved height is almost ten metres. Near the basin is an ancient gate, that was called Arco di Santa Maria in the Middle Ages. Through this wall a fortress was created within the city wall, namely the area to the south of the hexagon.
The gate. Testaguzza 1970, p. 208.