San Pedro church is believed to be located over a previous edifice housing the remains of the Córdoban martyrs Januarius, Martial and Faustus, dating to the 4th century AD. After the conquest of the city by king Ferdinand III of Castile (1236), a church dedicated to St. Paul was built here in his program of construction to give a Christian appeal to the previously Muslim city. Construction began in the late 13th century and was completed in the early 14th century.
The edifice's current appearance date mostly to later restorations. Part of the bell tower and two of the medieval gates have survived, a new one having been added by architect Hernán Ruiz II in 1542. In 2006, the church was elevated to the status of minor basilica by Pope Benedict XVI.
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.