Portus continued to be expand. In the late second century, there was both an expansion of existing buildings and the building of a new complex – The Grande Magazzini Di Septimo Severus. It was built adjacent to the Palazzo Imperiale. This building is two stories high, the ground floor consisting of a large corridor flanked by secure storage rooms whilst the layout of the first floor suggests offices. It has been suggested by some that this building had some sort of customs function as it overlooked both basins and so was ideally positioned to monitor ship movements.
The evidence seems to suggest that at this time there was a large increase in the trade in marble. Analysis of the remnants found suggest that this was primarily from Tunisia and Egypt. Once unloaded from the ships, this was probably transferred to a large marble yard situated at the junction of the Fossa Triana and the River Tiber, from where it was transferred as required to the Statio Marmorum in the city.
Despite the crises of the third and early fourth century Roman Empire it seems as though. Portus remained an active port. Within two years of Constantine’s adoption of Christianity, it is recorded that there was a Bishop of Portus. Sometime in the period 334-341 it was granted the status of a civitas. This is interesting as it implies there was an attached urban community, the location of which so far has remained undiscovered by excavation, although civil buildings such as a large basilica have been found. One intriguing snippet of historical evidence is a reference to the building of a guesthouse at Portus for the use of pilgrims by a senator Pammachus in 398. Were these pilgrims going to Rome or pilgrims going from Rome to the holy land or somewhere else? The Theodosian law code of 438 contains multiple references to Portus regarding the control of state-owned warehouses and the charges for the import and export cargoes
it appears that Portus went into decline in the fifth century, no doubt as a result of the decline of the influence of Rome itself as power moved to Constantinople in the East and to places such as Ravenna in the West. Despite this, it remained an important port and there is a record of one Sidonius Apollimarus being appointed as the urban prefect for Portus in the mid-5th century. From the late fifth century, there appears to have been a contraction of the port area and we see the construction of new defences centred on and around the hexagonal basin. Little is known of what happened after the fall of the Roman kingdom in Italy in 489. However, we do know that during the Byzantine –Ostrogoth war of 535-553 Portus is recorded by a Byzantine historian Procopus as being an important and strategic prize and it seems following the Byzantine victory Portus continues to function as a port. Archaeological evidence suggests that the Claudian basin was by now silting up and there seems to have been no political will to dredge it. Also from around the late fifth century onwards burials begin to be found in areas outside that of the hexagonal basin, suggesting that these areas were no longer in use for port activities. It has been suggested that the Byzantine port consisted just of the Darsena and the hexagonal basin and that these were accessed via the canal system or that a limited path was dredged through the Claudian basin.
It may be that despite the decline of the ports usage the town continued to be of some importance as there seems to be evidence of a refurbishment of the basilica in the sixth and seventh centuries. The last historical record of use for the port is when in 715 the fleet of Pope Constantine is recorded as sailing from Portus