Ratiaria, originally conquered by the Dacians and later by the Romans, was known for its gold mine possibly exploited by Thracians. The Romans became involved around 75 BC, and by 29 BC, Ratiaria was a fortified city. Under Augustus, the Romans officially conquered the region, organizing it into the province of Moesia. Ratiaria became a key station with a legionary fortress and fleet. After the division of Moesia in 86 AD, it became the capital of Upper Moesia.
Legio IV Flavia Felix was stationed here until the conquest of Dacia (101-106 AD). Afterward, Ratiaria became a colonia named Colonia Ulpia Traiana Ratiaria (107 AD). In the 2nd and 3rd centuries, it prospered as a trade center. With the definitive abandonment of Dacia Traiana in 271, old castra were reopened. By 285, Ratiaria served as the capital of Dacia Ripensis, hosting the military governor and the Roman legion XIII Gemina. It later became an important Christian center in the 4th century, with recorded bishops and theologians. The city faced sackings by the Huns in 440-441 and the Avars in AD 586. Archaeological excavations started in 1958, revealing its historical significance.
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.