Founded to defend the Tiber river
Tradition says that Ostia was founded by Ancus Marcius, the 4th king of Rome, who lived during the second half of the 7th century B.C., even if - till today - there is little archeological evidence corroborating such information. However, it seems that during the Regal period there was a built-up area near the mouth of the Tiber, where there were some salt marshes producing salt: a priceless and essential wealth. Salt was used to flavor and preserve food, so, keeping its production under control was absolutely necessary, as the historian Titus Livius says, recalling, among other things, that the whole area surrounding the mouth of the Tiber was strategically important for Rome.
In any case, the first settlement can be traced back to the beginning of the 4th century B.C., immediately after the defeat of the Etruscan town of Veio, situated on the right bank of the river, which fell to the Roman army in 396 B.C. Only at the end of that century a squared fortified post (castrum) was built. This castrum was surrounded by strong tufa walls and its main road axes - the Cardus and the Decumanus - were north-south oriented and east-west oriented, respectively. This military camp, called Ostia from the Latin word Ostium meaning "mouth of the river", was established at a distance of around 16 miles from Rome, as a military outpost to keep under control not only the access to the Tiber, but also its lower course and nearby areas, in order to defend Rome.
Ostia, the first Roman colony, became immediately a river port acquiring a commercial function to supply Rome with food stuff, particularly wheat, even if its strategic military function as naval base certainly prevailed.
During the first period, the political control exercised by Rome over Ostia had been very strict, but towards the end of the Republican period the city became more autonomous, as demonstrated by the presence of a stable government body, the Council of Decurions, which independently promulgated public acts. The city was slowly spreading far beyond the perimeter of the castrum, where originally only 300 families lived. At that time, population had increased and could no longer be contained in the small fortified post, so the city slowly changed: from a strategic military outpost it turned into the commercial port of Rome; this function eventually determined its new urban aspect.
The city was then surrounded by a new circle of walls, approximately 2 km longer, that is traditionally attributed at the time of Sulla, even if recent studies suggest that it was built in the mid 1st century B.C. on the initiative of Cicero. The walls circumscribed an urban area of about 50 hectares, divided into five regions or neighborhoods. The city could be accessed through three main gates: Porta Romana, that is the Eastern gate at the end of Via Ostiense, Porta Laurentina that is the Southern gate at the end of the Cardus Maximus, and Porta Marina, that is the Western gate, facing the sea, at the end of the Decumanus Maximus.