From the colourful houses in the village of Bosa, travelling one kilometre along the Temo river, you will reach the locality of Calamedia, a site that was already inhabited in the Phoenician-Punic age: here, you will find the cathedral of San Pietro, the most ancient Romanesque church on the island. Its red trachyte walls stand near the river banks, on the opposite shore of the hill of Serravalle, dominated by the castle of Malaspina, inside of which you will find another gem of religious architecture: Nostra Signora di Regnos Altos (Our Lady of Regnos Altos). The cathedral of San Pietro is said to be extra muros because it is located outside the castle walls and it is more than half a century older than the castle itself. Around it, there was the original nucleus of the town, which was inhabited until the end of the sixteenth century. When the district of Sa Costa was later completed on the hillside, the population moved. The migration took two centuries: Bosa vetus disappeared.