"Ostia" (plural of "ostium") is a derivation of "os", the Latin word for "mouth". At the mouth of the River Tiber, Ostia was Rome's seaport, but due to silting the site now lies 3 kilometres (2 miles) from the sea. The site is noted for the excellent preservation of its ancient buildings, magnificent frescoes and impressive mosaics.

The location of the main sector of the city, directly connected to the Tiber river and the river port, led to the development of a warehouse district from the Republican period onwards; initially these were constructed only in the area south of the decumanus since the area north of the road, used to load and unload goods, could not be built on at the time, as known from the boundary stones of Caninius. Starting from the early imperial period, large warehouse complexes were also erected in the area north of the decumanus as part of a larger urban planning project that also involved the construction of public buildings like the Caserma dei Vigili (barracks of the fire-brigade), baths and private houses overlooking the street with monumental porticoes. There are also numerous warehouses in the other districts of the city, especially in the area next to the Tiber, forming part of an urban fabric that was also diversified from a functional point of view.

The travertine boundary stones that took their name from the praetor Gaius Caninius delimited an area of public land between the Tiber and the decumanus in the eastern sector of the city. Set aside for the unloading, transportaion and storage of goods, a prohibition against the construction of private buildings was in force in the area. The boundary stones date to the late Republican period, around 140 BCE, and are evidence of the enlargement of the port of Ostia in connection with the economic and social reforms of the Gracchi. The gradual raising of the street level and the occupation of this strip of public land by the complexes of imperial period led to the boundary stones definitively losing their function.

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Ostia Antica, traditionally founded in the late 4th century BCE, possibly under the reign of Ancus Marcius, its early function was military, securing Rome’s access to the sea and controlling salt production. By the mid-Republican period, it evolved into a vital commercial hub, facilitating the movement of goods, particularly grain from Sicily and Africa, into Rome. Ostia saw significant expansion under the emperors Claudius (reigned 41–54 CE) and Trajan (reigned 98–117 CE), with the construction of new harbors and storage facilities, including the massive hexagonal port at Portus. By the 2nd century CE, the city had developed into a densely populated urban center with a well-planned street grid, multi-story insulae (apartment buildings), public baths, temples, and a theater, reflecting its economic and social significance within the empire.

The decline of Ostia began in the later imperial period due to a combination of factors, including the silting of the Tiber, changes in commercial routes, and the growing prominence of Portus Romae, which became the primary harbor. By the 4th and 5th centuries CE, the city experienced depopulation, although it retained some administrative and religious functions. The abandonment accelerated after the fall of the Western Roman Empire in 476 CE, and by the early medieval period, much of Ostia was in ruins. Its buildings were gradually quarried for stone, and the site was largely buried by riverine sediment over the centuries. Excavations beginning in the late 19th and early 20th centuries CE revealed a remarkably well-preserved urban landscape, offering critical insights into Roman urbanism, trade, and daily life.

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Ostia Antica's civic and public structures were essential to the city's administrative, social, and cultural life, reflecting the well-organized urban planning of a major Roman port. The forum, the heart of civic activity, was surrounded by key governmental and religious buildings where official decisions, legal proceedings, and public announcements took place. Nearby, the Barracks of the Fire Brigade (Caserma dei Vigili) housed the city's firefighting corps, a crucial institution for maintaining order and responding to the dangers of densely built, multi-story insulae. This barracks also contained the Augusteum, a shrine dedicated to the imperial cult, underscoring the role of public institutions in reinforcing state ideology.

Bathing complexes were among the most prominent public structures in Ostia, serving not only hygienic but also social and recreational purposes. The Forum Baths, Baths of Neptune, Baths of the Carriage Drivers, and Baths of the Marina Gate, among others, varied in size and decoration but typically featured a sequence of hot, warm, and cold rooms, along with exercise areas and elaborate mosaic floors. The Baths of Neptune, named after their impressive black-and-white mosaic depicting the god Neptune, exemplify the high artistic and engineering standards of Ostian bath architecture. Other baths, such as the Baths of the Seven Sages and the Baths of the Envious One, provide insight into the social stratification of the city, with some catering to specific professional groups or neighborhoods.

Theatre and public porticoes also played a significant role in Ostian civic life. The Theatre of Ostia, originally built under Augustus and later expanded in the Severan period, accommodated thousands of spectators for performances, public speeches, and possibly religious ceremonies. Adjacent to the theatre, the Plaza of the Corporations (Piazza delle Corporazioni) once housed offices of trade guilds, demonstrating the integration of entertainment and economic functions within public spaces. The Eastern and Western Porticus of Pius IX with Loggias provided covered walkways for pedestrians, offering shelter from the sun and rain while serving as gathering places for daily interactions.

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The infrastructure of Ostia Antica was a highly developed and organized system that supported the daily life and commercial activities of Rome’s primary port city. Its streets followed a structured urban layout, with the Decumanus Maximus, the main east-west road, intersecting with the Cardo Maximus, the principal north-south axis. These paved roads were lined with raised sidewalks and had embedded drainage channels to manage rainwater and waste. Water supply was ensured through an extensive network of aqueducts, lead pipes (fistulae), and cisterns, which distributed water to public fountains, baths, and private residences. The city’s sanitation system included latrines, such as the large public vespasianae, which were strategically located near baths and markets, equipped with running water for flushing. Ostia’s public baths, including the Forum Baths and Baths of Neptune, relied on sophisticated heating systems like hypocausts and large reservoirs to provide heated water. Storage and distribution of goods were facilitated by massive horrea (warehouses), including the Grandi Horrea, which stored grain supplies essential for Rome. This well-integrated infrastructure not only sustained Ostia’s population but also played a crucial role in the city’s function as a key maritime and commercial hub of the Roman Empire.

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The residential structures or houses in Ostia Antica reflect the architectural and social diversity of the city's inhabitants, ranging from modest multi-story insulae (apartment buildings) to more elaborate domus (private houses). As a major port city of Rome, Ostia saw the development of dense urban housing to accommodate its commercial and working-class population, with insulae providing compact living spaces often arranged around central courtyards. These buildings, constructed primarily of brick-faced concrete, featured multiple floors with shops or storage areas on the ground level and living quarters above. In contrast, wealthier residents occupied larger domus, characterized by spacious layouts, decorative frescoes, mosaic floors, and private courtyards or gardens. Notable examples include the House of Serapis, the House of the Paintings, and the House of Jupiter and Ganymede, which display intricate interior ornamentation and architectural refinement. Additionally, some structures, such as the Guild Seat of Trajan and the House, suggest a blending of residential and professional functions. The variety of housing in Ostia illustrates the city's well-planned urban landscape, reflecting both the practical needs and social stratification of its diverse population.

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The religious and cultic structures of Ostia Antica reflect the city’s diverse spiritual landscape, shaped by its role as Rome’s principal port and a hub of cultural exchange. These structures, ranging from state-sponsored temples to private shrines and foreign cult sanctuaries, illustrate the intersection of official Roman religion, imperial worship, and the presence of mystery cults and non-Roman faiths. Major temples, such as those dedicated to Jupiter, Hercules, and Rome and Augustus, dominated the civic and political core of the city, reinforcing state ideology. Meanwhile, sanctuaries devoted to deities like Mithras, Serapis, and Isis highlight the increasing prominence of Eastern cults during the Imperial period. The presence of a synagogue, one of the earliest known in the western Mediterranean, attests to the religious diversity within Ostia’s population. Today, the archaeological remains of these structures provide critical insight into the evolution of religious practices in the Roman world, revealing how Ostia’s inhabitants engaged in both public and private acts of devotion.

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The large number of commercial and industrial structures in Ostia Antica reflect the city’s pivotal role as Rome’s principal port, facilitating trade, production, and economic activity on a vast scale. As a thriving commercial hub, Ostia housed warehouses (horrea) for the storage of grain, oil, and imported goods, such as the Grand Warehouse and the Warehouse of Epagathus and Epaphroditus, which played a crucial role in supplying Rome. Markets and specialized shops, including the Shop of the Fish-Seller, provided essential goods to the local population and transient merchants. Industrial production was also well-represented, with facilities like the Mill/Bakery of Silvanus, which processed and distributed grain-based products, and the Fullonica on the Augustan Road, which served the textile industry through laundering and dyeing operations. The Plaza of Corporations, a key center of trade, was lined with offices of shipping companies and guilds responsible for managing Ostia’s commercial exchanges. These structures, interconnected through Ostia’s well-developed infrastructure, illustrate the complexity of the city’s economy and its function as an economic lifeline for Rome. The architectural remains of these buildings provide valuable insight into the daily operations of commerce and industry in the Roman world.

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Roman Gate
The Roman Gate (Porta Romana), was the main entrance to Ostia along the Via Ostiensis, dating to the first century BCE. Excavated in 1911, it was originally at a much lower level than today. Constructed with large tufa blocks, it featured two interior rooms, grooves for wooden doors, and flanking square towers (each six meters wide). Large inscriptions once placed high on either side of the gate recorded its construction by Marcus Tullius Cicero and completion by his rival Publius Clodius. A later Domitianic restoration (81–96 CE) raised its level, rebuilt the structure, and added marble decorations, now displayed nearby. A colossal statue of Minerva-Victoria, originally placed atop the gate, was discovered in the early 20th century; the original is in the museum, while a copy stands near the gate. Nearby, the Cippus of Salus Augusti, a marble statue base (1.20 x 1.20 x 1.05 meters), was found in 1910 CE. Dedicated by Glabrio, a member of the Acilii Glabriones family, it honored the well-being of the emperor and was likely set up during an imperial visit, possibly Claudius. Additionally, a marble sundial was unearthed near the gate in 1858.

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Marina Gate
The Marine Gate (Porta Marina), part of Ostia's first-century BCE city walls, derives its modern name from the nearby shoreline. Excavated between 1938 and 1942 CE, it consists of a single chamber constructed from large tufa blocks. On its southern side, grooves for wooden doors and pivot holes remain intact. Flanking the gate are two square towers, each measuring 4.40 to 4.50 meters per side. In the first century CE, the gate was completely dismantled. Later, between 210 and 235 CE, the "Bar of Alexander and Helix" (IV, VII, 4) was constructed to its east. To compensate for the lost entrance, a new arch, adorned with marble, was erected five meters south of the original gate, though only a few traces survive. A medieval lime kiln was also discovered in the space between the original gate and the later arch.

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Imperial Palace
The so-called Imperial Palace (Palazzo Imperiale) of Ostia Antica is a multifunctional complex located at the western end of Region I, excavated primarily between 1855 and 1871 and later investigated in the 1980s. Despite its name, there is no clear evidence that it served as a palace, though stamped lead pipes suggest a connection to Matidia the Younger, possibly indicating an imperial donation. The structure evolved over several centuries, incorporating baths, commercial spaces, residential units, and religious areas, including a mithraeum and a shrine to Silvanus.

The earliest sections, possibly a Hadrianic warehouse (117–138 CE), were later expanded. By 145–150 CE, public baths were constructed, featuring a palaestra, a frigidarium with a large pool, a caldarium, and decorative black-and-white and polychrome mosaics, many of which were later removed to the Vatican Museums. Notable mosaics include charioteers with named horses, maritime scenes, and personifications of the seasons. During Commodus’ reign (180–192 CE), additional courtyards, apartments, and shops were built, and by the Severan period (193–235 CE), more commercial spaces and structural reinforcements were added. Further modifications in the third century CE suggest continued adaptation, including rebuilding entrances, reinforcing piers, and adding staircases. The complex, resembling Insula III,X, functioned as a large-scale mixed-use facility, integrating commerce, housing, and public amenities within Ostia's urban landscape.

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Lamp Fountain
The Fountain with Lamp (Fontana con Lucerna), a fine marble fountain, stood along the Decumanus in front of the long porticobuilt in the Hadrianic period (circa 117-138CE) to monumentalize the street. There was probably already a basin lined with hydraulic plaster on this spot from the Augustanperiod (late first century BCE to early first century CE); the fountain was raised in the Hadrianic period to match the new street level and lined with marble in thethird century CE. The fountain took the form of a basin decorated with a screen and completely covered in marble, with columns that may have supported a pergola. At the centre of the basin was a little pillar whose top was shaped like and oil lamp from which water gushed forth.

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Republican Fortress
The Republican era fortress (Castrum Repubblicano) is the oldest part of the city of Ostia to be documented archaeologically; it dates to the fourth century BCE. It is a fortified settlement surrounded by walls in tufa blocks with four gates, located at the ends of the main streets (the cardo and decumanus). Other streets at therightangles subdivided the internal space into plots of regular shape occupied by temples, public buildings and especially houses. ALready in the third century BCE,the settlement stretched outside the walls, which had lost their original defensive function; Rome's expansion the Mediterranean had gradually reduced the city's military role and simultaneously increased its commercial vocation.

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The Archaeological Museum of Ostia Antica houses a rich collection of artifacts excavated from the ancient port city, providing valuable insight into the daily life, commerce, and religious practices of Ostia’s inhabitants. Established in the 19th century, the museum was created to preserve and display significant statues, inscriptions, mosaics, and frescoes found across the site, many of which were removed from their original locations for protection.

The museum’s collection includes statues of deities such as Mithras, Venus, and Neptune, busts of emperors and prominent citizens, elaborate sarcophagi, and funerary reliefs. Notable exhibits feature intricately decorated household items, trading inscriptions, and a variety of sculptures that once adorned public spaces and private residences. Some pieces come from the Forum, the Baths of Neptune, the Theatre, and the many mithraea of Ostia. The museum plays a crucial role in conserving Ostia Antica’s artistic and historical heritage, offering visitors a deeper understanding of the city’s importance as Rome’s primary port and a vibrant economic hub in antiquity.