Soon after the Erkelenz was elevated to a town status in the 12th century, work began on the brick fortifications of the place. These probably consisted of basic ramparts as had been common since time immemorial for the defence of settlements. Although the castle was not documented until 1349, the town appeared to have developed under the protection of the castle.
The castle has 23 high keep, curtain walls and corner towers. It was part of larger city walls. It was badly damaged by French army in 1674.
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.