In the third century AD, the city of Vannes, called Darioritum, acquires the right to fortify. Thus, west of the Gallo-Roman city, a castrum was built. During the Middle Ages, the castrum becomes the centre of the city. Extended in the 14th century, it is reinforced in the 15th century. Over the following two centuries, the ramparts are modernised with the construction of the Garenne buttress. Several medieval gates has been survived, like Tour du Connétable and Château de l'Hermine (former castle, transformed into a palace in the 17th century). Today there is a nice view from the park Jardins des Remparts to the walls.
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.