In the 12th and 13th century, Näs (or Visingö) Castle on the southern end of Visingsö was the residence for the fragile Swedish monarchy. It was built probably by Sverker I of Sweden (died in 1156) or his son Charles VII of Sweden (Karl Sverkersson). Näs was thereby the oldest Royal castle in Sweden. It was a residence 5-6 kings before in 1318 Birger I (Birger Magnusson) pawned it to Danish. The castle was probably demolished soon after.
References: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.