Aquileia was founded by the Romans as a military colony in 181 BC in a place that was at the intersection of people and trade. He was first bulwark against the invasion of the Barbarians and the departure point for expeditions and military conquests.
Connected by a good road network, over time it became increasingly important for its trade and for the development of a highly refined craftsmanship. It reached its peak during the reign of Caesar Augustus: with a stable population of over 200,000 inhabitants, became one of the largest and richest cities throughout the empire. It was the residence of several emperors, with a palace very popular, until Constantine the Great and beyond.
When he arrived the Christian message (the tradition speaks of a coming of St. Mark the Evangelist, who went to Rome to do consecrate S. Ermacora from St. Peter as the first bishop of Aquileia), it was rapidly developing underground, so as to explode promptly upon public worship was granted the Edict of Milan in 313 AD
Suffice to say that were erected promptly three large classrooms, luxurious, placed together in a horseshoe: two main, parallel to each other, joined by a cross. Each could comfortably hold two to three thousand people: something unthinkable for a simple “start” of evangelization and for the huge resources needed to achieve them. These soon afterwards were insufficient to contain all the faithful, and had to be demolished to make way for other larger classrooms. In fact we find that, a few decades later (about 345), starting from the foundations of the House North, was erected a much larger (over 70 meters long and a width of 31: 5 feet longer than the one we see), the largest ever for Aquileia: that in 452 AD was destroyed by Attila and ever more resources. Even the South Hall, expanded under Bishop Chromatius was partially destroyed by the invasion of the Huns. At this point it should be noted a characteristic and unique Aquileia all the various basilicas were strictly rectangular and without apse.
When the children of the survivors and exiles returned to Aquileia and thought of a reconstruction, turned their attention to the residual structures of the House South, which still was extended in length and width: will the foundations of the latter to act as a support, after a long period of complete abandonment (by the Lombards 800), the construction of a true church, as we understand, and that is roughly the perimeter of the current one. Quest ‘work was completed by the bishop Maxentius (811-838), with the financial help of Charlemagne. Subsequently, however, before the Hungarians and then an earthquake (988) made it unusable. Remnants of the mosaic floor of this basilica can be explored through two hatches: one at the altar at the center of the chancel and the other at the tomb of St. Peter.