Not far from the left hydrographical bank of the Temo River, the cathedral of St. Peter, called extra muros, was built in the early medieval period, flanked by a Roman-based tower/bell tower. The valley was inhabited without interruption from prehistoric times until the end of the 16th century, when the Sa Costa district was completed on the slopes of the dominant Serravalle hill (an internal migration that lasted at least two centuries: Bosa vetus disappeared). Archaeological excavations have ascertained, in addition to the presence in the plain of erratic pieces from the Roman period, the existence of a fenced cemetery on the northern flank of the building. Slabs with inscriptions walled in on the outside and preserved on the inside (many missing) testify to Bishop Constantine de Castra as the builder around the middle of the year 1000, but examination of the masonry certifies the existence of an earlier structure.
The collapse due to river flooding compromised the apsidal block to such an extent that, in the 1940s/1950s, it was painstakingly rebuilt, restoring the complex to its medieval appearance, disfigured by continuous incongruous ‘restorations. Today we admire an isolated church of Romanesque-Gothic appearance, which has lost its title of cathedral to a building at the foot of the hill.
Trined on square pillars, it is devoid of ancient liturgical furnishings (some of which are housed in the local Curia). The intervention of Cistercian masters (documented in the area) is evident in the tripartite facade, decorated with three wide ogival arches and interlaced arch decoration, at the apex of which rises a shrine with knotted columns, not earlier than the late 13th-early 14th century.
A rare detail is the presence of a tall salvaged lintel on the facade portal. Of controversial dating, it nevertheless shows that it has undergone rework to bring it up to date with thirteenth-century fashion (see Jesus’ bonnet). It depicts (starting from the left): St. Paul (with sword: he was beheaded), Constantine haloed (his sanctity is recognized only in the Orthodox world), Our Lady and Child, Peter (with the keys to Heaven).
