Kastell Biriciana was a former Roman military camp. It was occupied from the 1th century AD to the mid-3rd century as a part of Limes Germanicus (a line of frontier fortifications that bounded the ancient Roman provinces). There was first a wooden fortifications, which were later replaced with a square stone fort. The known internal buildings are marked by stone slabs. There is today a reconstructed north gate, the large thermal baths and the Roman museum.
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.