The chapel or lower crypt, dedicated to St. Leocadia of Toledo, a martyr who was especially revered in the era of the Visigoths, was destined to safeguard under its altar the remains of two martyrs of Córdoba, Sts. Eulogio and Leocricia, which were brought to Oviedo in January 884, during the tenure of Bishop Hermenegildus I (868-891) and reign of Alfonso III “the Great” (866-910). Although it was initially conceived as a crypt for the martyrs, it was later used as a pantheon for the bishops.

The upper chapel, which was traditionally considered a palace chapel connected to the palatial buildings attached to the Cathedral, might have served as a private chapel for the bishops, separate from the ceremonies related with the Basilica of the Holy Saviour, and would then have been part of the buildings for the clerics in the service of the Cathedral.

The dedication to St. Michael the Archangel has come down to us from the 12th century, at which time it underwent a profound transformation.

Toward the last third of that century, once the pilgrimage to the shrine of St. James in Compostela had evolved and the veneration of the relics of the Arca Santa (Holy Ark) had spread, it was subjected to a process of reform of its function, structure and decoration. From that time on, it served as a reliquary chapel. The original roof with barrel vault supported on two transverse arches was replaced; a triumphal arch was built in front of the sanctuary area, and the previous altar was demolished in order to set up the Arca Santa (Holy Ark) in its place.

The most characteristic feature is clearly the images of the Twelve Apostles sculpted in the shafts of the columns that support the transverse arches. They appear in pairs: on the north wall, St. Simon and St. Jude, St. James and St. John, St. Andrew and St. Matthew; on the south wall, St. Thomas and St. Bartholomew, St. Peter and St. Paul, St. James the Less and St. Philip.

At the foot of the nave, there was a Calvary, but only the sculpted heads of Christ, Mary and St. John have been conserved, which had formerly been completed with the remains of the figurative elements painted on the wall, but which are no longer extant.

An iconographic repertory that, taken as a whole, alludes to the salvation of man through the Redemption and subsequent Resurrection of the Lord, disseminated throughout the world by the apostles.

The Treasures and Relics of the Cathedral of Oviedo are conserved in the sanctuary of this chapel, which include: the Holy Shroud and the Holy Ark (11th century), the Cross of the Angels (880), the Cross of Victory (908) and the Agate Box (910), among many other relics, constituting one of the most significant collections of the High Middle Ages in Spain and in Europe.

The Cathedral of Oviedo has received the name of Sancta Ovetensis thanks to the number and importance of the relics safeguarded in the Cámara Santa.

The bombing of the Cámara Santa during the revolutionary events of 1934 had a severe impact on the building. The restoration work took place between 1939 and 1942: Manuel Gómez Moreno and Alejandro Ferrant were involved in the excavation and survey of the ruins, and Luis Menéndez-Pidal y Álvarez and Víctor Hevia Granda, in their commendable restoration.