The vast plain in which the rivers flowing down from the opposite sides of the Alps and the Apennines ran, constituted the congenial environment par excellence for inland navigation in all eras, using suitable boats. Forty years of geomorphological and topographical studies have shown that from the pre-Roman age to the mid-twelfth century the main course of the Po flowed through the site where the Byzantine castrum of Ferrara was built in the sixth century AD. Ferrara enjoyed an undisputed privileged position in the early Middle Ages, being the only urban center on the river and being located at the root of the Po di Volano and Po di Primaro branches of the Po, which then constituted the main delta branches of the Po. thus it became the obligatory point of passage between the sea and the Po inland. Ferrara had three ports overlooking the river. This is explicitly documented by the Statutes of Ferrara of 1287. Still on the bank of the Po di Ferrara, but further downstream, in the branch wedge of the Primaro dal Volano, the fair was held twice a year, lasting fifteen days, which attracted Italians and foreigners, in spring on Palm Sunday (in Ferrara degli Olivi) and in autumn on the feast of St. Martin. The mercantile wealth of Ferrara therefore arose from the favorable and undisturbed confluence of many waterways.