Ostia may have been Rome's first colonia. According to legend, Ancus Marcius, the fourth king of Rome, was the first to destroy Ficana, an ancient town that was only 17 km (11 mi) from Rome and had a small harbour on the Tiber, and then proceeded with establishing the new colony 10 km (6 mi) further west and closer to the sea coast. An inscription seems to confirm the establishment of the old castrum of Ostia in the 7th century BC. The oldest archaeological remains so far discovered date back to only the 4th century BC. The most ancient buildings currently visible are from the 3rd century BC, notably the Castrum (military camp); of a slightly later date is the Capitolium (temple of Jupiter, Juno and Minerva). The opus quadratum of the walls of the original castrum at Ostia provide important evidence for the building techniques that were employed in Roman urbanisation during the period of the Middle Republic.
Ostia was a scene of fighting during the period of the civil wars between Gaius Marius and Sulla during the 1st century BC. In 87 BC, Marius attacked the city in order to cut off the flow of trade to Rome. Forces led by Lucius Cornelius Cinna, Gnaeus Papirius Carbo and Quintus Sertorius crossed the Tiber at three points before capturing the city and plundering it. After his victory here, Marius moved on to attack and capture Antium, Aricia and Lanuvium to further destroy the foodstores of Rome.
Sacking by pirates
In 68 BC, the town was sacked by pirates. During the sack, the port was set on fire, the consular war fleet was destroyed, and two prominent senators were kidnapped. This attack caused such panic in Rome that Pompey the Great arranged for the tribune Aulus Gabinius to rise in the Roman Forum and propose a law, the lex Gabinia, to allow Pompey to raise an army and destroy the pirates. Within a year, the pirates had been defeated.
The town was then re-built, and provided with protective walls by the statesman and orator Marcus Tullius Cicero.
During Julius Caesar's time as Dictator, one of his improvements to the city was his establishment of better supervision of the supply of grain to Rome. He proposed better access to grain by the use of a new harbour in Ostia along with a canal from Tarracina.
The town was further developed during the first century AD under the influence of Tiberius, who ordered the building of the town's first Forum. The town was also soon enriched by the construction of a new harbour on the northern mouths of the Tiber (which reaches the sea with a larger mouth in Ostia, Fiumara Grande, and a narrower one near to the current Fiumicino International Airport). The new harbor, not surprisingly called Portus, from the Latin for "harbour", was excavated from the ground at the orders of the emperor Claudius. This harbour became silted up and needed to be supplemented later by a harbour built by Trajan finished in the year 113 AD; it has a hexagonal form, in order to reduce the erosive forces of the waves. Moreover, at a relatively short distance, there was also the harbour of Civitavecchia (Centum Cellae). These elements took business away from Ostia itself and began its commercial decline. In 2008 British archaeologists discovered the remains of the widest canal ever built by the Romans, 300 feet wide, which they believe ran Portus across the Isola Sacra to the Tiber opposite Ostia, which would have made the transport of large quantities of goods far easier than by land transport. In 2014 ruins on the north side of the river opposite the city were discovered indicating a large built-up area with some massive structure. Ostia within the walls cover an area of 69 hectares or 173 acres. During the 4th century city spilled over the southern walls to the sea south of Regioni III and IV on the map.
Ostia itself was provided with all the services a town of the time could require; in particular, a famous lighthouse. The popularity of the Cult of Mithras is evident in the discovery of eighteen mithraea. Archaeologists have also discovered the public latrinae, organised for collective use as a series of seats that allow us to imagine today that their function was also a social one. Ostia had a large theatre, many public baths (such as the Thermae Gavii Maximi, or Baths at Ostia), numerous taverns and inns and a firefighting service. Ostia also contained the Ostia Synagogue, the earliest synagogue yet identified in Europe; it created a stir when it was unearthed in 1960-61.
Late-Roman and sub-Roman Ostia
Ostia grew to 50,000 inhabitants in the 2nd century, reaching a peak of some 100,000 inhabitants in the 2nd and 3rd centuries AD. Ostia became an episcopal see as part of the Diocesi of Rome as early as the 3rd century AD; the cathedral (titulus) of Santa Aurea being located on the burial site of St Monica, mother of Augustine; she died here in 387 in a house property of the Diocesi of Rome.
In time mercantile activities became focused on Portus instead. For scholars of the High Empire Ostia was the seaside version of Rome, the city of apartment buildings. It used to be thought that the city entered a period of slow decline after Constantine I made Portus a municipality, Ostia thereby ceasing to be an active port and instead becoming a popular country retreat for rich aristocrats from Rome. In spite of the fact that Portus shows substantial growth in the 4th century, the traditional view that Ostia went into marked decline has had to be revised due to recent excavations and re-evaluation of the evidence. The knocking down of some apartment blocks replaced by houses of the rich was "thought to have signalled the disappearance of Ostia's once-vibrant group of non-elite residents and labourers"..."recent research has suggested we take a more nuanced view of residential patterns and social demography in the Late Antique city". Earlier views of decay relied on fleeting references in the ancient sources and excavators ignoring evidence from the period that the town continued to thrive despite pockets of decay into the 6th century, "..life in Ostia ended not with a Vesuvian bang but with a whimper" after a slow decline from the 6th to the 9th centuries.
The city housed the headquarters of the Prefect of the Annona and his large staff. Although there are signs of decay in certain quarters, evidence indicates continued prosperity through fifth century, including: repairs on baths (26 remained in operation during the 4th century), public buildings, church construction, street repaving, residential and business expansion beyond the perimeter of the south wall (the presence of a small harbour, the Porta Marina on the sea, is attested), a huge 4th century villa located east of the Maritime baths, and the continued operation of the river port on the western edge of the town, the 'navalia', a squarish basin built in from the river, a warehouse on the east side and, behind it, a large bath complex, erroneously called the palazzo imperial. Numerous bathing establishments are recorded as still operating in the 4th and into the 5th centuries with major repairs of the center-city Neptune Baths in the 370s.
The city was mentioned by St Augustine when he passed there in the late 4th century. The poet Rutilius Namatianus reported the lack of maintenance of the city ports in 414 AD. This view has been challenged by Boin, who states Namatianus' verse is a literary construct and not consistent with the archaeological record, ibid. pp. 22, 25, (the poet was lamenting the lost greatness of Rome after the sack of 410 and was hoping for the rise again of the great city).
After the end of the Western Roman Empire in 476 (traditional date: Julius Nepos died 480 was the last legitimate emperor), Ostia fell slowly into decay as the population of Rome, 700-800,000 in A.D. 400 contracted to 200,000 or less in 500 A.D. The city was finally abandoned in the 9th century due to the repeated invasions and sackings by Arab pirates. A naval battle, the Battle of Ostia, was fought there in 849 between Christians and Saracens; the remaining inhabitants moved to Gregoriopolis a short distance away.
Sacking and excavation
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A "local sacking" was carried out by Baroque architects, who used the remains as a sort of marble storehouse for the palazzi they were building in Rome.
Soon after, foreign explorers came in search of ancient statues and objects.
The Papacy started organising its own investigations with Pope Pius VII; under Benito Mussolini massive excavations were undertaken from 1939 to 1942 during which several remains, particularly from the republican period, were brought to light. The first volume of the official series Scavi di Ostia appeared in 1954; it was devoted to a topography of the town by Italo Gismondi and after a hiatus the research still continues today. Though untouched areas adjacent to the original excavations were left undisturbed awaiting a more precise dating of Roman pottery types, the "Baths of the Swimmer", named for the mosaic figure in the apodyterium, were meticulously excavated, in 1966–70 and 1974–75, in part as a training ground for young archaeologists and in part to establish a laboratory of well-understood finds as a teaching aid. It has been estimated that two-thirds of the ancient town are as yet unexcavated. In 2014, a geophysical survey using magnetometry, among other techniques, revealed the existence of a boundary wall on the north side of the Tiber enclosing an unexcavated area of the city containing three massive warehouses.