Nendrum Monastery may have been founded in the 5th century, but this is uncertain. The monastery came to an end at some time between 974 and 1178, but its church served a parish until the site was abandoned in the 15th century. Some remains of the monastery can still be seen.
Dendrochronology has dated a tide mill on the island to the year 619, making this the oldest excavated tide mill anywhere in the world. The monastic site included orchards, gardens, pastures, arable fields, and a guest-house.
The principal monastic remains which can now be seen are three concentric cashels (enclosures) of dry stone walling, but these were substantially rebuilt by Lawlor in the 1920s. The central cashel has the round tower remains, a ruined church with a sun-dial, and a graveyard. The second cashel contains what is called a 'monastic school' or workshop and other burials.
The canonical sundial now seen at one corner of the ruined church was reconstructed from fragments found during the excavation of the site in 1924 and has been dated to about the year 900. One of only a few early medieval sun-dials known to exist, it takes the form of a vertical stone pillar, 190 cm high, 40 cm wide and 15 cm thick, with the dial and gnomon on one face at the top. However, because of the nature of the reconstruction, the original height of the pillar is conjectural.
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.