Legend has it that the Apostle Peter landed at this place, then on the coast, on his way to Rome. The church which he founded here, the Ecclesia ad Gradus (Church by the Steps), became from an early period an important staging-point for pilgrims making their way to Rome from the north. The present aisled Romanesque basilica, under which excavation has revealed remains of an earlier church, probably dates from the 11th century.
The choir apse at the east end is flanked by two subsidiary apses, and there is also an apse at the west end; there is no transept. The church has frescoes of about 1300 (lower register, portraits of Popes; middle register, scenes from the life of St Peter; upper register, the heavenly Jerusalem) which are generally attributed to Deodato Orlandi.
