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 Château du Lude is one of the many great châteaux of the Loire Valley in France. Le Lude is the most northerly château of the Loire Valley and one of the last important historic castles in France, still inhabited by the same family for the last 260 years. The château is testimony to four centuries of French architecture, as a stronghold transformed into an elegant house during the Renaissance and the 18th century. The monument is located in the valley of Le Loir. Its gardens have evolved throughout the centuries.