The first building on the site of current Sion Cathedral was built in the 9th century and destroyed by fire in 1010. The next Romanesque cathedral existed until the 15th century. The existing bell tower dates from the 13th century. The nave was rebuilt in Gothic style between 1450 and 1500. The cathedral contains several Roman inscriptions, 15 altars, and many fonts.
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.