The basilica of St. Plechelm is a Roman Catholic church dedicated to the 8th-century Irish monk Saint Plechelm, whose festival on 15 July has been on the calendar of the medieval diocese of Utrecht ever since his canonisation in the 10th century.
The oldest parts of the existing building date from the middle and the second half of the 12th century, but the history of the church goes back to the 8th century, when the traveling missionary Plechelmus founded the first church on the site, the Silvesterkerk, initially dedicated to pope Silvester and sometime before 954 rededicated to Plechelmus himself (canonised after the elevation of his relics).
In 954, bishop Balderik of Utrecht founded a chapter in Oldenzaal and expanded and restored the 8th-century church, allowing the relics of Plechelmus used on his canonisation to be transferred to it. The current tower was erected around 1240. The church was enlarged in Gothic style in 1480s.
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.