Since the beginning of the VI century A.D. many refugees from Oderzo, who escaped the continuous invasions from the barbarians, found shelter in the isle of Equilio, founding the city of Jesolo.
To a slightly previous age dates, in fact, the small palaeochristian basilica of which we only have a few ruins left, that can be found today in the Antiche Mura area. From then on, Jesolo, thanks to its trades and the ideal insular position that protected it from the barbarians'assaults, saw a constant flourishing.
In the beginning of the IX century it was so busy that it ordered the Benedictine Monastery of S. Giorgio in Pineto to be built in 819 as well as a big Cathedral dedicated to S. Maria, of which the ruins are visible in the old area of the city. To witness the ever growing importance of Jesolo as political and religious centre, or simply as defence post, is the fact that from 864 to 1466 it was the seat of the bishop's palace. In 899, after yet another barbarians'assault (the Ungari), the big Santa Maria Cathedral and the Benedictine Monastery of S. Giorgio in Pineto went destroyed.
A century later another Monastery was added to the ones of San Giorgio in Pineto and San Mauro (which was built in the area in the VII century, too), namely the one in Torre di Caligo wanted, according to the sources, by San Romualdo, already founder of the Camaldolesi Order. The devastating effects of the 1100 sea-quake meant the decline of Jesolo's importance, reducing it to little more than a village, in such a way that when in 1487, twenty years after the abolition of the bishop's palace (1466), the Patriarch Matteo Girardi visited Jesolo for the first time, he did not mention the Cathedral at all. Perhaps it was already a ruin.