The section (sector M) runs from the ancient porta Ostiensus (today Porta San Paolo) to the Tiber River, including a large part of the riverside alluvial plain known today as Testaccio, from Mons Testaceus. Today, much of the walls’ course in its south-west portion is hard to see as much of the section that ran along the Tiber was removed to make way for the industrialisation of the district from the late 19th century onwards.
The original route can be partially reconstructed with the help of historical maps. From the gate, the walls veered south-west in the direction of the Tiber, with a course of about 900 m long, at the beginning of which was the pyramid tomb of Caius Cestius and a postern gate, today extensively altered. A number of inscriptions and a column commemorating episodes related to the Resistance at Porta San Paolo in the days after 8 September 1943 have been erected here.
In this section, it is possible to reconstruct the presence of 22 towers. In its present state, the wall is mainly preserved in its mediaeval and post-medieval reconstructions. The one carried out under Pope Nicholas V (1447-1455) is particularly striking and monumental, clearly identifiable by the papal coats of arms in the high towers.
It is possible to walk the exterior of the entire stretch along Via di Campo Boario and on the interior in the monumental Non-Catholic and Commonwealth Cemeteries. At the river banks, the walls curved abruptly towards the north, forming a right angle to align with the course of the Tiber, which they followed for about another 900 m, and then continued on the right bank, at the porta Portuensis, which no longer exists.
The point where the walls turned onto the bank was fortified by an angular tower connected to a section of wall that ran transversally towards the Tiber, ending with another tower to control river access.
Only one tower remains of this section, visible on the Testaccio embankment and heavily altered in the Middle Ages.