The Castle is located at the entrance of the town, though detached from its ramparts, and it houses the Town Hall and the Municipal Museum.

Its keep, leaning on a part of the original wall (14th century), was built from 1424 onwards. The visitors have access to the watch turrets through the museum. The town’s flag, blue with a white cross and a red quarter brocaded with the ermine taken from the town’s coat of arms (see further, St Vincent’s Gate), flies constantly from the top of the building, as a reminder of the former privileges of Saint Malo, which was an autonomous town between 1590 and 1594.

The two biggest towers facing inwards date back to the last third of the 15th century. Next to Saint Thomas’ Gate, the “Quic-en-Groigne” Tower owes its name to an inscription which was erased during the French Revolution. It recalled that the tower was erected by order of Anne de Bretagne, even though the inhabitants of Saint Malo were against it: “Quic-en-Groigne, ainsi sera, c’est mon plaisir,” meaning “whatever your opinion may be, this tower will be built.” The inside of the inner courtyard, with its ancient barracks, and the view on the imposing keep are quite picturesque.