Isola Sacra - Central south part - Defensive wall

A defensive wall can be traced clearly from the present bank of the Tiber in the east, running in a line for c. 345 m. to the west. The wall is about 3-5 m. thick and has three rectangular external towers (6 x 8 m.) placed about 80 m. apart. A probable fourth tower lies partly beneath the trackway a further 74 m. to the west. To the east, the wall has clearly been cut by the river when it changed course in the 16th century. To the west the wall changed course, turning to run north-south. Given the character of this wall and especially the provision of external towers, there can be no doubt that this is a town wall delimiting the settlement on this side of the Tiber.

There are significant differences between the Isola Sacra wall and the late-republican city-wall of Ostia. We could then think of a late-antique date and a link with the late-antique wall of Portus, especially the internal wall with the Arco di Santa Maria. It should be noted that Procopius, writing about events in c. AD 570, notes that Ostia was "without walls", but that statement could be related to the bulk of the city, to the south of the Tiber.

Plan of buildings 1-5 and the defensive wall. Image: Keay 2020, fig. 6.6.