 It was suffragan of the Metropolitan Archdiocese of Sassari.The bishopric dates back to Byzantine times (circa 1000 AD), but the earliest mention of a bishop of Castro is of 1116, when an unnamed bishop of the see assisted at the dedication of the Basilica di Saccargia.In 1164, its bishop Atto dedicated a church in the locality of Aneleto and granted it in the following year to Camaldolese monks.