Vestiges ... (détruit depuis 1255)

Aleth Cathedral was established in the 9th c., then destroyed by Norman invaders in the 10th c. but later rebuilt. The site of Aleth however was not a secure one and the town of St-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.

Competition with the new town came to a head in 1255, when resentment in Aleth at the tax demands of St-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.