Today, Aquileia is a town smaller than the colony first founded by Rome. Over the centuries, sieges, earthquakes, floods, and pillaging of the ancient buildings for materials means that no edifices of the Roman period remain above ground. The site of Aquileia, believed to be the largest Roman city yet to be excavated, is inscribed on the UNESCO World Heritage List.
Excavations, however, have revealed some of the layout of the Roman town such as a segment of a street, the north-west angle of the town walls, the river port, and the former locations of baths, of an amphitheater, of a Circus, of a cemetery, of the Via Sacra, of the forum, and of a market. The National Archaeological Museum contains over 2,000 inscriptions, statues and other antiquities, mosaics, as well as glasses of local production and a numismatics collection.
The most striking remains of the Roman city are those of the port installations, a long row of warehouses and quays that stretch along the bank of the river. These were incorporated into the 4th century defences, substantial traces of which can be seen today.
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.