The relics of the Apostle James and his disciples, Athanasius and Theodorus, are kept in the Roman mausoleum below the Cathedral of Santiago’s high altar. They are housed in a 19th-c. silver urn, whose shape imitates a Romanesque sarcophagus.
To go down to this small crypt, you should head to the Cathedral’s ambulatory. Once inside, the itinerary begins by going up to the apostle’s alcove, at the top of the high altar, where it is customary to embrace the saint, represented by a 13th-c. Romanesque statue. Due to sanitary restrictions by Covid19, this tradition is not currently allowed.
You then go down to the mausoleum and briefly visit the urn; there is not a lot of room inside.