Supported by an imposing round arch, its walls are rich with various frescoes, the oldest of which date back to the 13th century and were brought to light in 1937: Foot Washing and Last Supper (left wall), Annunciation (on the archway), Passion, Christ in Glory and Christ the Judge (beyond the archway), St. Narno and St. Viatore, first city bishops, and St. Alexander on horseback (mullioned window and lunette).