The earliest known document of Onsenoort Castle dates from 1286. The first castle was badly damaged and restored before 1372 in the border wars between local landlords. After it was changed hands several times the castle was again destroyed by French army in 1787. Today on the keep remains of a medieval castle. Current buildings were mainly built or restored in the late 1800s and early 1900s.
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.