The ancient city of Ialyssos extended around the hill of Filerimos, which was the ancient acropolis where there are remains of buildings from the Archaic, Byzantine and Knights' periods. The temple of Athena Polias, which dates to the 3rd-2nd centuries BCE, was built over the site of an earlier Classical temple, to judge from from the evidence of a 5th century BC floor and terra-cotta antefixes found there. The depository on the west side produced pottery and votive offerings dating from the 9th to the 5th centuries BC.
In addition to the cult of Athena Polias, in Ialyssos there are also references to a cult of Zeus Polias. In the Early Christian period (5th-6th centuries AD) a three-aisled basilica with an atrium was built on the remains of the ancient temple, in the north aisle of which a single-aisled church with a cupola was constructed in the 10th century.
In the time of the Franks this site was occupied by a medieval monastery and church.
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.