Around 450 there was a Christian municipality in the Roman luvavum with two churches and a monastery. After the withdrawal of the Roman military in 488, the city was destroyed during the Migration Period.
In 696 Holy Rupert came and founded the monastery Saint Peter and the nunnery on the mount Nonnberg. Around 700 he probably built a big church in honour of Holy Peter and established a community of world priests (canons) there, where the chapter evolved later on.
In 739, Bonifatius gave the papal confirmation for the recently founded diocese and fixed the diocesan borders. On April 20, 798, Salzburg was elevated by pontifex Leo III to archbishopric at the request of the Frank king Charlemagne to which the Bavarian suffragan dioceses Freising, Neuburg, Passau, Regensburg and Säben were subordinate. This ecclesiastical province temporarily enclosed the whole Old-Bavarian tribal area, thus, the bigger part of today’s Austria and Bavaria (except for Franconia and Swabia), today’s South Tyrol and Trentino, big parts of Hungary, Czechia, Slovenia and Slovakia. Arno was the first archbishop.
In the Investiture Conflict the archbishops of Salzburg were on the side of the pontifex. Under Gebhard the city of Salzburg was destroyed several times by followers of the emperor. Hence, for protection purposes, Gebhard built the fortress Hohensalzburg in 1077.
In 1167, the cathedral also burnt down in the course of a city fire and the cathedral treasury was destroyed. Konrad III – the first cardinal of Salzburg – could begin with the reconstruction work after the conclusion of peace in 1177 and built a splendid three-nave Romanesque cathedral.