Santa Maria del Regno church, together with the annexed castle of which ruins remain today, was built in the 11th century by Giorgia, daughter of the Giudice of Torres, as a Palace Chapel. The church is mentioned in the Libellus Judicum Turritanorum of the 13th century.
The church was built in dark basalt stone by Pisan workers (the island in the Middle Ages was under strong influence from the maritime Republic of Pisa). The façade is divided into five sectors and has a salient-shaped façade. In the middle is the portal, surmounted by a double mullioned window. The whole exterior of the edifice is characterized by false columns (lesenes) and Lombard bands; on the right are the remains of the square bell tower, which is missing the upper part.
The interior, on a rectangular plan, has a nave and two aisles divided by columns whose capitals have flower motifs. The nave has a wooden trusses ceiling, while the aisles are groin vaulted. In the semi-circular apse is the large retablo, the largest 16th-century polyptych in Sardinia, located behind the high altar. The table portrays several prophets and saints, as well as episodes in the life of the Virgin Mary. In the middle, within a niche, is the wooden statue of Nostra Signora del Regno, a 'Madonna with Child' wearing royal symbols. The polyptych is dated 1515.
The church's columns have 17th-century paintings with Apostles and other Saints, while also present is a lesser retablo from the same school, a carved wooden pulpit and an epigraph celebrating the consecration of the church on May 7, 1107.
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.