Emperor Claudius constructed the first harbour on the Portus site, 4 km (2+1⁄2 mi) north of Ostia, enclosing an area of 250 hectares (617 acres), with two long curving moles projecting into the sea, and an artificial island, bearing a lighthouse, in the centre of the space between them.

It passed the grove of the Arval Brothers at the sixth mile, to the Campus salinarum romanarum, the saltmarsh on the right bank from which it derived its name.

[3] In AD 103, Trajan constructed another harbour farther inland — a hexagonal basin enclosing an area of 39 hectares (97 acres).

[4] In 2010, "one of the biggest canals ever built by the Romans" was discovered in Portus, in an ancient port increasingly being seen as important as Carthage or Alexandria.

[1] Its abandonment dates from the partial silting up of the right arm of the Tiber in the Middle Ages, which restored to Ostia what little traffic was left.

To the west of the harbour is the cathedral of Saint Rufina, 10th century, but modernized except for the campanile, and the episcopal palace, fortified in the Middle Ages, and containing a number of ancient inscriptions from the site.

On the island Isola Sacra just opposite is the church of S. Ippolito, built on the site of a Roman building, with a picturesque medieval campanile (13th century ?

[1] The division between the ancient settlement and the medieval Porto began in the 4th century AD, when Constantine had a line of walls built.

[citation needed] Ostia, just opposite on the left bank of the Tiber, was increasingly depopulated after Vandal and Saracen attacks.

[2] As part of Parco Archeologico di Ostia Antica the remains of Porto are open every Thursday, the first and the third Sunday of the month from 9:30 to 13:30, and upon request and advance booking at other times.