Along the left bank of the river Temo, stands the church of San Pietro Extra Muros in Bosa, considered the oldest Romanesque church in Sardinia. Built in red trachyte between 1062 and 1073, this imposing building served as the town's cathedral and bishop's seat in the past. The façade, attributed to the architect Anselmo da Como, is distinguished by three four-lobed rose windows and pointed arches. The portal is surmounted by a valuable limestone architrave, on which are depicted, from right to left, St Peter, the Madonna and Child, St Constantine and St Paul. The interior of the church has three naves: the central one has a wooden roof, while the side naves have cross vaults. Of particular historical interest is a Latin epigraph preserved to the right of the main entrance, documenting the laying of the church's foundation stone, a ceremony celebrated by Bishop Constantine de Castra, probably in 1053.