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:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.