The cathedral, located in the heart of the Borgo Antico next to the Bishop’s Palace, dominates a square square surrounded by low houses. Dedicated to St. Mary of the Purification and St. Bass, the city’s patron saint, it dates from the 12th-13th centuries and is attributed to Alfano da Termoli. It stands on the remains of an 11th-century church that was in turn built on an earlier temple dedicated to Castor and Pollux.
The east-facing, salient facade has two distinct levels: the lower, original section and the upper, rebuilt after the 1456 earthquake. Differences between the two sections are visible in materials and style. The portal is adorned with a lunette with the Purification and sculptural decorations depicting animals and human figures. The lower facade has seven arches, of which the central one is wider, and shows oriental influences. The upper part includes a large splayed oculus and bands decorated with foliate motifs. The right flank features twelve blind arches with a secondary doorway.
The interior shows two building phases: the lower church, partially preserved, and the upper, Romanesque-Gothic church, with a basilica plan with three naves without a transept. The aisles end in apses decorated with hanging arches and slender columns. The floor of the upper church is raised due to the presence of the ancient building below. The nave has a truss roof, while the side aisles have cross vaults.
To the left is the Chapel of the Blessed Sacrament, connected to the Bishop’s Palace. The third bay leads to the crypt, which preserves the sarcophagus of the remains of St. Bassus, sculptural fragments of the ancient facade, mosaics with zoomorphic figures, and a tombstone from 1238, once covering the well where the relics of St. Timothy were found.
Highlights include several altars: that of Mary Most Holy of the Rosary, Most Holy Savior, and St. Sebastian. In the right apse is the altar of St. Bass with a grotto, where the saint’s sarcophagus was discovered.
Also found in the crypt was a reliquary with the skull of St. Timothy, now kept in the church dedicated to him.
The square bell tower with octagonal cusped cell and sacristy complete the structure, making the cathedral a significant example of Romanesque-Gothic architecture.