The Basilica of Saints John and Paul on the Caelian Hill was built in 398 AD over the home of two Roman soldiers, John and Paul, martyred under the emperor Julian in 362. The church was thus called the Titulus Pammachii and is recorded as such in the acts of the synod held by Pope Symmachus in 499.
The church was damaged during the sack by Alaric I (410) and because of an earthquake (442), restored by Pope Paschal I (824), sacked again by the Normans (1084), and again restored, with the addition of a monastery and a bell tower.
The church has three naves, with pillars joined to the original columns. The altar is built over a bath, which holds the remains of the two martyrs. The apse is frescoed with Christ in Glory (1588) by Cristoforo Roncalli. Below this fresco are three paintings: a Martyrdom of St John, a Martyrdom of St Paul, and the Conversion of Terenziano (1726) by Giovanni Domenico Piastrini, Giacomo Triga, and Pietro Andrea Barbieri.
During excavations performed in the 19th century, a series of Ancient Roman rooms were discovered under the nave of the church. Some of these rooms date back to the first and fourth centuries AD. In one room an elegant third-century AD fresco depicting Proserpine and other divinities among cherubs in a boat can be found, as can traces of another marine fresco and mosaics in the window arches.
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.