A city wall enclosed a number of buildings around Trajan's basin. Existing buildings were incorporated in the wall. The wall has in the past often been attributed to the reign of Constantine, but recent research shows that it was erected c. 400 AD, just before the invasion of the Goths in 408 AD, or even later. It was restored extensively. There were square protruding towers, every 30 metres, but sometimes also at irregular distances, when towers had been added later.
Stretch nr. 5 of the wall is to the west of the Porticus of Claudius (52) and runs along the basin of Claudius. The wall has here been preserved up to a height of six metres.