The Cathedral of San Panfilo stands a short walk north of the old town centre, on the spot where (according to tradition) a Roman temple to Apollo and Vesta once stood. The current building dates mainly from the 11th to 13th centuries, with Gothic and later additions; the 2009 earthquake forced a long restoration that has now reopened the church to visitors.
The most interesting feature is the crypt: a low, vaulted Romanesque space with carved capitals and the relics of San Panfilo, the 7th-century bishop who is Sulmona's patron. The main church above is austere by Italian standards, with a wooden ceiling and an impressive Baroque organ. Free entry; respect service times.