Alghero was designated as a diocesan seat in 1503 but construction work on the cathedral did not begin until 1567. It was inaugurated in 1593 but was not finished. After several restorations it was consecrated in 1730.
The church was originally in Catalan-Gothic style, as can be seen in the five chapels and ambulatory of the presbytery, which also includes the octagonal base of the bell tower. The nave and the two aisles are however in Late Renaissance style. The main altar was designed by the Genovese artist Giuseppe Massetti (1727): the sculpture shows Mary the Immaculate flanked by angels. He also designed the ambulatory and the pulpit. In 1862 a Neo-Classical narthex was added to the façade, which dramatically changed its appearance.
The first chapel on the right side is dedicated to the Blessed Sacrament. Its imposing, marble altar was inaugurated in 1824. It is located inside a circular temple, reminding the Temple of Vesta in Rome.
The cathedral is the burial site of the Italian-born Duke of Montferrat (1762-1799) and his brother Count of Asti (1766-1802) who died on the island having caught malaria. The marble mausoleum was sculpted by Felice Festa in the early 19th century.
References:The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.