It is no coincidence that the development of numerous medieval towns owes much to their location at the intersection of land routes and navigeable waterways. While the Romans (no lovers of maritime travel) favoured roads, in the Early Middle Ages there was a shift in preference towards water-based routes, which affected settlement patterns and established the roots of many future towns. Whether a town prospered and grew, or stagnated, depended in part upon its ability to compete with other regional settlements as a market centre; the better its connections to other trading centres in other parts of the country or overseas, the more likely it was to flourish. Mobility was important to the medieval merchant. Matthew Paris' map of Britain (mid-13th century) showed the principal rivers as abnormally prominent features, and most of the towns or castles on the map were located either on the coast or on one or other of the rivers; the importance of rivers continued to be highlighted on later maps, including the Gough map of mid-14th century. This importance was due not only to their transportation role, but to roles as sources of fresh water (despite their opposing role as sewers) and sources of power for mills.
The River Wensum flowed in one direction into the Yare, which in turn provided quick access to the sea and thereby to coastal trade (and other rivers flowing into northern Norfolk and Suffolk), to foreign ports, and to the fishing-grounds of the North Sea. It nourished not only commerce but also excellent meadow-lands for grazing livestock, while it and the streams that ran off it fed corn-fields and served the development of local industries such as (at different periods) the manufacture of pottery and the finishing of cloth.
In addition the river provided an obstacle to hostile forces, making it unnecessary to extend city walls along the southeastern stretch of the city, where the Wensum acted as the boundary. On the other hand, for boats the river provided an easy access through the defensive lines – too easy. Consequently, where the river entered the city from the south, the local authorities built, where the city walls terminated a tower on each bank of the river (that on the west is shown here); between these were hung "two great chains of good Spanish iron across the river with the machines wound by a windlas in the tower on the west so that no ship nor barge nor boat might come in or depart without leave, nor against the will of those who have to govern the city." [Hudson and Tingey, The Records of the City of Norwich, Norwich: Jarrold, 1910, vol.2, p.218] York was using chains for a similar purpose on the Ouse in 1380.
