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.
Visby Cathedral (also known as St. Mary’s Church) is the only survived medieval church in Visby. It was originally built for German merchants and inaugurated in 1225. Around the year 1350 the church was enlarged and converted into a basilica. The two-storey magazine was also added then above the nave as a warehouse for merchants.
Following the Reformation, the church was transformed into a parish church for the town of Visby. All other churches were abandoned. Shortly after the Reformation, in 1572, Gotland was made into its own Diocese, and the church designated its cathedral.
There is not much left of the original interior. The font is made of local red marble in the 13th century. The pulpit was made in Lübeck in 1684. There are 400 graves under the church floor.