The Cathedral of San Pietro Apostolo, seat of the Diocese of Isernia-Venafro, stands on foundations that archaeologists have traced through several building phases back to at least the early medieval period. The present neoclassical facade and interior date from rebuilding after the 1805 earthquake, which was the most recent of the catastrophic events that have repeatedly destroyed and rebuilt the city. Beneath the current structure, excavations have uncovered evidence of a Roman temple and layers of subsequent Christian construction.
The cathedral's modest exterior gives way to a spacious interior with side chapels containing baroque artworks that survived earlier earthquakes. The crypt preserves more clearly ancient fabric and is worth asking to visit if open.