St Decuman's Church is on a very early Christian site and is dedicated to Saint Decuman. It was the 'bishop-house' of the cantref of Penfro and one of the seven principal churches in Dyfed under medieval Welsh law. It is constructed of rubble stone under a slate roof which carries a bellcote. The nave and chancel are possibly 13th century and the transept, chapel and tower 14th century.
The font, in the north porch, is 12th or 13th century; the nave has a small Norman font. The church contains a number of tombs and memorials, including two recesses and wall-mounted coffin lids. The oldest dated memorial is a 1716 baroque monument to F Powell of Greenhill. The pulpit and lectern are 19th century, and windows range from the 19th and 20th centuries, as recently as 1960.
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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.