The system of fortifications set up along the Trieux was to function from the Norman invasions until the Catholic League Wars. The protection of the Trieux thus started at Bréhat with the fortress of Lavrec Island which kept watch to the east. Then every bend in the river was kept safe by a fortress: Castel-Yar, Coetmen, the castle of Lézardrieux, Botloï, Frynaudour, la Roche Jagu, Chateaulin-sur-Trieux, all of which communicated by smoke signals, right up until the Vauban period. Of all these fortresses the only still standing is la Roche Jagu. Built during the 11th or 12th centuries, they were dismantled after the Wars of Succession or the Catholic League Wars either by their owners or because they fell out of use.