The first known owner of Sundholma was Henrik Klauson Fleming, who moved the estate to the current site in the late 1400s. He built a fortified manor house which was surrounded by a moat (it is still visible in the park). The current main building was built in the 1770s and early 1800s. The stone basement remains probably from the original Fleming's castle manor.
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.