In 169 BC 1300 families were settled there as a reinforcement to the garrison. After the discovery of the goldfields near the modern Klagenfurt in 130 BC, it soon became a place of importance, not only owing to its strategic position, but as a centre of trade, especially in agricultural products.
It became a Roman municipium probably in 90 BC. The 4th century AD was the greatest period of Aquileia: it became a naval station, a mint was established, and the bishop obtained the rank of patriarch. A council held in the city in 381 was only the first of a series of Councils of Aquileia that were summoned over the centuries.
An imperial palace was built, in which emperors after Diocletian frequently resided and by the end of the 4th centur Ausonius enumerated it as the ninth among the great cities of the world, placing Rome, Mediolanum and Capua before it, and called it "moenibus et portu celeberrima." In 452 it was destroyed by Attila, though it continued to exist until the Lombard invasion of 568.
The seat of the patriarchate of Aquileia had been transferred to Udine in 1238, but returned in 1420 when Venice annexed the territory of Udine. It was finally suppressed in 1751 when the sees of Udine and Gorizia (Gorz) were established.