The magnificent marble altar with the relics of San Castrense, commissioned by the archbishop Ludovico II de Torres, is located at the centre of the rectangular Chapel of San Castrense. The altar has a parallelepiped shape with four polychrome pillars, with small yellow marble towers on a red background, the symbol of Torres, in the centre of each. AD DOMINI ALTARE MEMINERITIS MEI is written on the floor under the altar.