The Cattolica was built in the 9th century, when Calabria was part of the Byzantine Empire. The name derives from the Greek word katholiki, which referred to the churches provided with a baptistery. It is one of the most important examples of Byzantine architecture, together with the church of San Marco in Rossano Calabro.
The Cattolica follow a plan with 'inscribed cross', typical of the middle Byzantine age. The interior is divided by four columns into five similar spaces. The square central space and the angled ones are covered by domes. The angled ones have tambours with the same diameter, while the central dome is slightly taller and larger.
The western sides lies on free rocks, while the southern area, ending with three apses, stands on three stone bases. The construction is in bricks.
The interior was once entirely covered with frescoes. The left apse has a bell built in 1577, when the church was converted to the Latin rite. The interior has also several inscriptions in Arabic, which have led scholars to suspect it could have been also used as a Muslim oratory.
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.