St. Mary's Church (Mariakyrkan) is the oldest still used building in Sigtuna. The brick-made church was constructed in the mid-13th century and inaugurated in 1247. It was, however, completed probably in 1255, when the archbishop Jarler was buried there. The church was enlarged and sacristy added in the 1280’s. Due the Reformation King Gustav Vasa ordered to demolish the adjacent abbey in 1530 and St. Mary's became a parish church.
The full restoration was made in the 1640’s and the present was added then. Mural paintings were restored between 1904-1905. There are couple medieval artifacts remaining in the church, like fonts, a triumph crucifix and small altarpiece.
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.