St Mary's Church was built inside Alabum, a Roman fort, on top of Llanfair Hill. A monastic cell from around 1126 was already on the site before building commenced. It is one of the largest medieval churches in the county, and has a substantial tower at the west end with a square stair-tower on its northeastern corner. The church is built of rubble stone with plain tiles on the roof. Earlier dressings are mostly in red sandstone, while nineteenth century dressings are in Bath stone. The nave and chancel are directly connected to the tower, to the porch on the south side and to the vestry and organ chamber on the north. The hymn writer William Williams Pantycelyn (1717–1791) is buried in the churchyard, where there is a memorial to him.
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.