The chapel was commissioned by Archbishop Ludovico II Torres to house the relics of St. Castrense (patron of the city) and to make it the place of his own burial.
It is accessible from the right aisle. It was built starting from 1590 and reflects a sober Renaissance taste in the style. The plant is rectangular.
The marble altar that preserves the relics of San Castrense is surmounted by a canopy that fits well under the large masonry dome.
Supported by four polychrome marble columns, surmounted by white Corinthian-style capitals, the canopy is painted with the dove of the Holy Spirit, surrounded by small cherub heads and four full-length angels holding the symbols of the passion.
