Inside the Archiepiscopal Museum there is the Oratory of St. Andrea, built by Pietro II Bishop in full Theodorican period. Before coming to the chapel we pass through a narthex of rectangular plan with barrel vault, the decoration represents a flowered lawn with hens, doves, ducks, peacocks and pheasants. In the walls of the narthex there is a long Latin inscription (twenty hexameters) in mosaic and says: "aut lux hic nata est, aut capta hic libera regnat". It expresses the polichrome luminosity of the mosaic rooms, vibrating of reflexes and of unimaginable change of colours. In the lunette above the door a Christ warrior that tramps onto a lion and a snake the symbols of evil annulled bu the divine force. On the book that Christ holds in his hand is written: "Ego sum via, veritas, et vita". In the cupola of the chapel  decorated with mosaic there are four angels there are the symbols of the Evangelists, in the intradoses of the four arches at the sides of the central vault, there are the mosaics of Christ, Apostles and Saints.