Passage: 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. The cathedral was consecrated by Pope Gelasius II in 1118, and from 1133 had archiepiscopal rank. After the fire of 1296, provoked by fights between Guelphs and Ghibellines, the building was partly restored and partly rebuilt. Between 1307 and 1312 the façade was completed, the inner colonnades rebuilt with capitals and matronei added. The Romanesque structures remained pretty untouched, and frescoes of religious subject were also added.

Question: In what year were the walls constructed?
1155