In the 9th century, a wooden castrum was built on a rocky promontory to fend off Viking raids near Durivum (Saint-Georges-de-Montaigu). Later, a stone keep was added with strategic upgrades, including a drawbridge. The Montaigu family, later known as Belleville, pledged loyalty to the Plantagenets. Over time, the property changed hands through marriages and alliances, including a notable period of privateering by Jeanne de Belleville. The fortifications of Château de Montaigu were enhanced in the 15th century under Louis XI's guidance to protect against Breton threats.
Some visible remnants still exist. Originally, there was an oval-shaped enclosure with about ten towers and an entrance gatehouse located in the northeast. The fortified town was situated to the north of the fortress and extended slightly to the east. A dry moat separated the fortified town from the fortress. A barbican protected the entrance gatehouse. A Romanesque keep of the Niort type, with its courtyard and a small enclosure, was located in the castle's courtyard, roughly in the middle in the north-south direction and facing west.
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.