Today, the city is small (about 3,500 inhabitants), but it was large and prominent in Antiquity as one of the world's largest cities with a population of 100,000 in the 2nd century AD.

Aquileia was founded as a colony by the Romans in 180/181 BC along the Natiso River . The colony served as a strategic frontier fortress  intended to protect the Veneti, faithful allies of Rome during the invasion of Hannibal. It was also important for the commercial trade of the Baltic amber (sucinum). The discovery of the gold fields near the modern Klagenfurt in 130 BC^[8] brought the growing colony into further notice, and it soon became a place of importance, not only owing to its strategic military position, but as a center of commerce, especially in agricultural products and viticulture. 

In terms of religion, the populace adopted the Roman pantheon, although the Celtic sungod, Belenus, had a large following. Jews practiced their ancestral religion and it was perhaps some of these Jews who became the first Christians. Meanwhile, soldiers brought the martial cult of Mithras. 

  In 168 Marcus Aurelius made Aquileia the principal fortress of the empire against the barbarians of the North and East .