The only remnant of a 13th century castle, the Tower of Saint-Clair is the testimony that Derval was in its time one of the strongholds of the Marches de Bretagne. In 1598, the Edict of Nantes condemned the castle, then fief of the ligueurs, to demolition. The almost complete destruction will not have been realized immediately, the fortress having had to serve as a prison for the Protestants captured during the siege of Port-Louis in Morbihan. This place will then serve as a quarry of stones for the people of Valais. Explanatory panels on site.