In the medieval mind, Yarmouth was associated with herring, a high-protein food important to the diet of the lower classes, which featured less meat than is eaten today. The thirteenth century seal of the borough bore depictions of a ship sailing herring-inhabited waters and, on the other side, St. Nicholas, a patron saint of fishermen. The fishery provided the reason for Yarmouth's foundation and the principal source of its medieval economy. 
Great Yarmouth -- the qualifier distinguishing it from its southern neighbour, Little Yarmouth -- is situated near where several rivers, among them the Yare, flow into what was once a very broad estuary opening out into the sea. In Roman times there was a port and market town a little further north, at Caistor, and a small fort at Burgh Castle; these were later abandoned. Subsequent settlement focused on the site of Great Yarmouth itself. Tradition has the first settlement there established by the Saxon leader, Cerdic, ca. 495, but this is unsubstantiated and doubtful. More certain is that silting in the mouth of the "Great Estuary" over time formed a huge sandbank that came to be several miles long, leaving the Yare access into the sea through two channels at either end of the sandbank; one channel separated Yarmouth and Caister, the other ran southwards for some miles and separated Great and Little Yarmouth/Gorleston before entering the sea. This sandbank eventually became firm enough to support dwellings, perhaps preceded by more temporary facilities for the drying, salting and smoking of herring, as well as the sale of herring. Fishermen from the Cinque Ports claimed a long-standing right to beach their boats and to dry their nets there. A fair may have been in operation there by the time of Edward the Confessor, during the forty days from Michaelmas to Martinmas when the fishery was at its peak; in later times this important fair attracted not only the Cinque Ports men, but also fishermen from the continent. The Cinque Ports had authority over the fair, through officers they appointed, which was subsequently resisted and then contested by Yarmouth. Another indication that Cinque Ports fishermen were likely among the founders of the town is that rents from some Yarmouth properties were due to the Ports. Yarmouth was a borough in the royal domain before and at the time of the Domesday survey, but an earlier shared jurisdiction is reflected in that Yarmouth had to pay every "third penny" of all public revenues to the earl. The number of burgesses living there suggests that its fishery was already important by this date, although Yarmouth was certainly a small town compared to Norwich or Ipswich, with a few hundred residents in all. 