St. John's Church was built between the 12th and 16th centuries, though there must have been churches preceding it. It was heavily damaged during World War II, but it has been restored. The oldest part of the building is the Romanesque tower. The tower dates from the early 13th century.
The choir, in part likewise from the 13th century, is a mixture of Romanesque and Gothic elements, and features two side chapels. To the left of the choir is the vestry, above which there is a room that must once have served as a chapel.
The 14th-century nave is now quite low, as it is covered by a modern ceiling inspired by that of the vestry.
The chapel to the south of the nave dates from the 15th and 16th centuries. Its two bays have Gothic vaults. The vaults of the northern aisle, which an inscription dates 1513, are flamboyant Gothic.
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.