During the second half of the 5th century, St. Severinus established a monastery here. The site was subject to repeated raids by the Alemanni. In 739, the recently consecrated English archbishop Boniface founded the diocese of Passau, which for many years was the largest diocese of the German Kingdom/Holy Roman Empire, covering territory in southern Bavaria and most of what is now Upper and Lower Austria. From the 10th century the bishops of Passau also exercised secular authority as Prince-Bishops in the immediate area around Passau (see ).