 The first cathedral, now the basilica of St. Syrus, was founded probably in the 5th or 6th century AD, devoted to Saint Sirus, bishop of Genoa.The transferring of the cathedral favored the urbanization of the zone that, with the construction of its walls in 1155, and the fusion of the three ancient city nuclei ("castrum", "civitas" and "burgus"), became the heart of the city.The piazza, in the absence of other public squares and centers of lay power, was the city's only public space for the whole of the Middle Ages.