The first forts of Legnago, erected along the two banks of the river Adige to defend the old river port and the river transport, date back to the tenth century; from that moment onwards, they marked the territory for about a thousand years. In 1405 Legnago was annexed to the Republic of Venice: in the first half of the fifteenth century Legnago and Porto were surrounded by walls and provided with two fortresses built by the river near the accesses to the bridge. In 1475 the Republic of Venice strengthened the fortress of Legnago by building four round towers at each corner: one of the four towers has survived so far.