Ostia

Roman tradition credited the founding of a city at the mouth of the Tiber to Ancus Marcius, the fourth king of Rome, and gave the year as 620 BC: Ennius says it was fortified and given a harbour, Livy ties it to the extension of Roman power to the sea and to the salt pans at the river mouth, and Cicero speaks of a colony founded there. Ostia may have been Rome's first colony. Nothing of that first settlement has been found on the site now visible, and it has been suggested that it stood elsewhere; an inscription has been read as confirming a castrum here in the seventh century BC, but the oldest archaeological remains so far discovered are of the fourth century BC.

The fortified camp from which the city grew was laid out during the fourth century BC, and its precise date is disputed: the earliest pottery from the castrum area is of 390–350 BC, while an excavation of 1971 on the line of the walls would date their building to the start of the third century BC.

Ostia probably began as a naval base. In 267 BC, during the First Punic War, it was the seat of the quaestor Ostiensis, who had charge of the fleet, and its commercial and military service in the Second Punic War won its inhabitants exemption from military service. Through the second century BC its role as the port through which grain was imported for Rome became the dominant one, and the praetor Gaius Caninius was sent to mark out a wide stretch of the left bank of the river as public ground for new quays, the old landing place on the right bank having become insufficient.

The civil wars reached the port. In 87 BC Marius, helped by his generals Cinna, Carbo and Sertorius, attacked Ostia in order to cut the flow of trade to Rome, captured it and plundered it. In 68 BC pirates sacked the town, set the port on fire, destroyed the consular war fleet and carried off two prominent senators; the alarm in Rome was such that Pompey the Great arranged for the tribune Aulus Gabinius to pass the lex Gabinia, allowing Pompey to raise an army against them, and within a year they had been defeated. The town was then rebuilt and walled, the circuit begun during Cicero's consulship in 63 BC and completed by Publius Clodius Pulcher, tribune of the plebs, in 58 BC. From 49 BC the city, until then governed directly by Roman magistrates, had magistrates of its own, recorded in the fasti ostienses.

Agrippa had the theatre built between 18 and 12 BC, the forum was probably laid out under Tiberius, and an aqueduct was built under Caligula. Claudius replaced the old office of the Ostian quaestors with an equestrian procurator annonae in charge of the grain supply, and, the harbour at Ostia being small, commissioned a new artificial harbour to the north at Portus; poorly protected from storms, it was supplemented by the hexagonal basin finished under Trajan in 113.

The port work brought prosperity, and the city kept its function as the administrative centre for the corn supply; it is estimated to have held around 50,000 inhabitants in the second century AD. Major building came under Hadrian, who twice held the chief city magistracy of duumvir, and continued at least to the end of the Severan age, when the theatre was restored and a large market and the Via Severiana were built. Eighteen mithraea have been found, a measure of how far the cult of Mithras had taken hold among the port's population, and the Ostia Synagogue is the earliest synagogue yet identified in Europe, probably built in the late first century AD and altered several times between the second and fourth centuries, including the installation of a Torah shrine.

The crisis of the third century cut the population, shown by buildings that collapsed and were never rebuilt, and the local magistracies disappeared as the city was placed directly under the authority of the annona. Constantine made Portus an independent city under the name Civitas Flavia Constantiniana and built an episcopal basilica at Ostia dedicated to Peter, Paul and John the Baptist, the town having become an episcopal see within the diocese of Rome as early as the third century. Economic activity largely moved to Portus, and rather than simply declining Ostia was turned into a luxurious residential centre, its production quarters abandoned and building resumed with much reuse of older material.

Augustine passed through in the late fourth century, and his mother Monica died at Ostia in 387 as they made their way back to Africa; the church of Santa Aurea was built on her burial site. Twenty-six baths were still in operation in the fourth century, and the Neptune Baths were substantially repaired in the 370s. Rutilius Namatianus wrote in 414 of the neglect of the city's ports, a view Boin has challenged as a literary construct inconsistent with the archaeological record. Prosperity in the fifth century is indicated by repairs to baths and public buildings, by church construction and street repaving, and by housing and business spreading beyond the southern wall.

By the end of the fifth century the aqueduct had stopped working. In 537, with the Goths holding Portus during their siege of Rome, Ostia became one of the strongpoints of the Roman defence commanded by Belisarius; the campaign succeeded, but by about 600 the human presence at Ostia and Portus had become negligible. Saracens sacked the place in 846 during their attack on Rome, and it was abandoned for Gregoriopolis, built a little to the east on the orders of Pope Gregory IV to shelter the few workers left in the district, the monuments of the old city being stripped for the new building. In 849 a further Muslim raid on the papal lands was halted off Ostia by the Campanian league under Caesarius of Naples. It was in consequence of the sack of 846 that Pope John VIII, who reigned from 872 to 882, walled Ostia and the basilica of San Paolo fuori le Mura to secure the communications between Rome and the sea.

In 1167 Pisans siding with the emperor and the antipope ravaged the coast as far as Ostia and sacked it, and when Richard I of England landed in 1190 he found immense ruins of ancient walls. The ruins were used for centuries as a quarry for marble for the palazzi of Rome, and Cosimo de' Medici had statues and jewels carried to Florence. Digging for saleable antiquities followed: the Portuguese ambassador to the Vatican sent statues, busts and floor mosaics to Portugal at the end of the 17th century and the Scottish painter Gavin Hamilton worked here in 1788, a trade that went on until Pius VII forbade it at the beginning of the 19th century. Antonio Nibby, who visited in 1819, found the surviving borgo down to about ten people in the malarial summer and a hundred in winter.

Giuseppe Petrini conducted the first excavations between 1801 and 1805 under Pius VII, and in 1855 Carlo Fea worked here with Pietro Ercole and Carlo Ludovico Visconti under Pius IX, who ruled that what was found should so far as possible be left on the site and set up the first museum in the old Casone del Sale. Under the Italian state Pietro Rosa dug from 1871 to 1874, Rodolfo Lanciani from 1877 to 1889, Dante Vaglieri from 1907 until his death in 1913, and Guido Calza from 1924 to 1946. Calza directed the extensive campaign of 1938–42, intended to present the new archaeological site at the Rome world exposition planned for 1942, which stopped when Italy became a major battlefield of the Second World War.

Two-thirds of the ancient town is estimated to be unexcavated still. A survey in 2014 found a boundary wall north of the Tiber enclosing an undug quarter with three massive warehouses, and in 2025 excavation uncovered what may be the oldest Jewish ritual bath found outside Israel, of the late fourth or early fifth century, within a large Roman house. The site is open to the public, and the finds are kept on it in the Museo Ostiense.