The history of Welzheim reaches back to the time of the Roman Empire. Around the year 160 AD the Romans founded a colony and two forts there; linguists reconstructed their Latin name as (Castra) Valentia on the base of the town's medieval name Wallenzin first mentioned in 1181. In 1980, for the 800th anniversary of the founding of the city, the eastern part (Ostkastell) of the ruins was reconstructed, and in 1993 an archeological park was founded on the site. It contains replicas of Roman stone monuments and explanatory plaques about Roman Welzheim. The western part of the fort was built over through the years.
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.