Raised in 1284-1288 during the signoria of Count Ugolino of Donoratico - whose coat of arms stands out in the inscription beside the entrance - this is the only Cathedral of Christianity ever dedicated to Saint Clare. Representing an evidence of the change from Romanesque to Gothic style in Sardinia, its façade is sided by a coeval bell tower of squared plan, guarding four bronze bells; the oldest (1327) is by A. Pisano.
Inside, transverse pointed arches divide the single nave into four bays; all of them (including the last one, above the presbytery area) are covered by cross vaults built in 1576-1588, enriched by fine works reflecting local st taste in the Spanish age. The 1 keystone is sculpted nd with a Crucifix; the gem of the 2 bay contains a scene from the martyrdom of St. Antioco (Patron of rd the diocese). In the 3 key is a figure in the round of a praying St. Antioco. The presbytery vault carries a figure of Christ the Judge and gems with Saints Clare, Sebastiano, Antioco and John the Evangelist.To the right of the entrance is an angel in white marble, carrying a holy-water font, th sculpted in 17 C. in late-Mannerist style. Between st nd the 1 and 2 bay, a gravestone on the floor indicates the burial of a cleric from Iglesias, Emanuele Marongiu, bishop of Nuoro until 1852
Two chapels were opened on each side of th the nave walls, starting from the 15 C. On the right side, the first chapel hosts a th 16 -C. simulacrum of San Nicola, bishop of Mira. The second, largely restyled in the th 19 C. and now dedicated to the Virgin of Carmelo, guards the tombstone of the bishop of Iglesias Giuseppe Delle Piane, died in 1920. On the opposite side, the first chapel (formerly dedicated to the Holiest Sacrament) hosts an altar in whose niche is the simulacrum of the Virgin of the Rosary, a wooden, partly damasked th polychrome statue, dating from mid-17 century. Dedicated to Saint John the Baptist, the second chapel on the left guards the remains of the bishop of Iglesias Giovanni Pirastru, died in 1978.
The present-day left transept results from th the enlargement of a small 16 -century chapel dedicated to Saint Antioco. The chapel was widened and embellished since 1656, but it was completed only in 1680, when the Ligurian architect Domenico Spotorno built its domed structure. In 1683, the Chapter of the Cathedral entrusted the Sicilian sculptor Onofrio Damatto with the building of an altar for this chapel, to be made in plaster and marble. His work was soon replaced by the installation of a great wooden altar completed in 1718, still impressive in all its grandeur.
The altar (11.00 x 7.70 m), made of carved and gilt juniper wood is structured in two orders: the lower one is marked by three niches enframed by four spiral columns; the upper one comprised between broken, jutting cornices, is divided into three panels by an imposing fastigium with a painting representing the Almighty (work attributed to Giacomo Altomonte), enframed by balustrades among vases, wreaths and festoons. The side panels of the second order host two pictorial compartments performed by Juan Maria Scaleta in 1718 featuring scenes from the life of Saint Antioco. In the central niche of the second order was a silver urn containing the relics of the Saint (until 1851). The side niches of the first order guarded the wooden statues of San Benedetto and SaintClare, while the wider, central niche shows an impressive statue of Saint th Antioco, of late 16 century.