SU 5520 is the western wall of a drainage channel bounded on the east by parallel wall SU 5519. At its northern limit it turns west 90 degrees, creating a T intersection with 5519 and 5518 to drain water from the west. In fact the channel cuts through the red tufo ashlar wall 5417 and extends further west bounded by SU 5535. It clearly belongs to a later phase of this area, probably associated with concrete walls.