Around 516, Saint Einion, king of Llyn, invited the Breton Saint Cadfan to move to the Bardsey Island from his first residence in Tywyn. Under Cadfan's guidance, St Mary's Abbey was built. In 1188, the abbey was still a local institution but, by 1212, it belonged to the Canons Regular. Many people still walk the journey to Aberdaron and Uwchmynydd each year in the footsteps of the saints, although today only ruins of the old abbey's 13th century bell tower remain. A Celtic cross amidst the ruins commemorates the 20,000 saints reputed to be buried on the island.
The Dissolution of the Lesser Monasteries Act of 1536, on the orders of Henry VIII, resulted in St Mary's Abbey being dissolved and its buildings demolished in 1537.
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.