Aleth was a Gallo-Roman settlement on a peninsula on one side of the Rance estuary. The bishopric was established in the 9th century. Aleth Cathedral (Cathédrale Saint-Pierre d'Aleth) was destroyed by Norman invaders in the 10th century but later rebuilt.
The site of Aleth however was not a secure one and the town of Saint-Malo had begun to grow up on a far more defensible site on a rocky islet in the estuary. In 1146 the seat of the bishopric of Aleth was moved to Saint-Malo by the bishop Jean de Châtillon. The bishopric thus became the Diocese of Saint-Malo and the abbey church Saint-Malo Cathedral, replacing the cathedral in Aleth.
Competition with the new town came to a head in 1255, when resentment in Aleth at the tax demands of Saint-Malo resulted in armed conflict. Aleth lost, and the walls, fortress and church were dismantled as punishment. The former cathedral ruins are still to be seen in the commune of Saint-Servan which occupies the area of the former Aleth.
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.