fter the defeat of the dispossessed Viscount Trencavel in 1240, when he tried to reconquest his city, the ramparts of Carcassonne were considerably strengthened, as well as five other fortresses of the region.
Carcassonne > History > Five sons
Carcassonne's Five Sons made up a tremendous line of defence in front of the Spanish border (Kingdom of Aragon at that time) : Aguilar, Quéribus, Peyrepertuse, Puilaurens and Termes. These fortresses still exist today, and are very famous as "The citadels of vertigo".
After centuries of rivalries and wars, France led by King Louis XIV (the Sun King) and Spain signed the Treaty of the Pyrenees in 1659. Louis married Infanta Maria Teresa and the Treaty gave France a new province : Roussillon.
The new frontier moved back on the crest line of the Pyrenees. The five fortresses lost at the same moment any strategic importance. Some of them kept a small garrison, sometimes till the French Revolution (1789-1799). But they slowly fell into neglect, becoming herdmen's or brigands' shelters.