The Martinikerk (Martin's church) is the oldest church in Groningen. The church and its associated tower (the Martinitoren) are named after Saint Martin of Tours (316–397), the patron saint of the Bishopric of Utrecht to which Groningen belonged. The church was a cathedral for a short period during the first bishopric of Groningen (1559–1594).
The origins of the Martinikerk are a cruciform church built in the 13th century, which was extended in the 15th and 16th centuries. It contains several 16th century tombs and Wessel Gansfort's 18th century tomb. Much of the wall and roof paintwork has been preserved. Of particular note is a 16th-century depiction of the life of Jesus Christ.
The tower was built from 1469 till 1482, with later additions. Citizens of Groningen often refer to the tower as d'Olle Grieze (Old Grey One). The original 13th century tower was destroyed by lightning and a new tower was built in the 15th century, also destroyed by lightning.
References:Since I can’t be there, maybe someone would be interested in taking a video of where the Organ is situated; then kindly uploading it to YouTube, for instance.
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.