It lies between the towns of Maury and Cucugnan, and is quite easy to get to from either. Maury is more accessible since it sits right on the D117 route, the major artery (meaning it's four lanes in places) through Cathar country just west of Perpignan. Plus, on your way back down from the castle, you can stop by Domaine Pouderoux for a taste of their wonderful (organic!) wines. Cucugnan is cradled in a valley between high mountain slopes, and so is harder to get to, and rather isolated, though if you stop there for lunch, you're bound to find some hearty Roussillon cooking.
Peyrepertuse is another of Carcassonne's "five sons", though this one fell to the French without a fight in 1258 when they gained the territory through their treaty wiht Catalonia and Aragon. No wonder, because in a battle, it would be impregnable. It is situated on one of the hills of the Corbieres, at an altitude of 800 meters, and the site has been occupied since Roman times. The first mention of Peyrepertuse as we know it was in 1020, when it was under control of the Counts of Barcelona, and then the Kings of Aragon.
In the 13th century, the castle's keeper was a man named Guillaume de Peyrepertuse, who was an Albigensian knight. He refused to submit to Catholicism, but eventually surrendered after the seige of Carcassonne in 1240, and handed it over to the French. When the French signed the treaty in 1258 with the Spanish, it changed the border of France and Spain to run right through the Corbieres for the next four centuries. Peyrepertuse lay right along it, and so the French built it up even further and kept it garrisoned to full strength. When the border with Spain was redrawn farther south in the Treaty of the Pyrenees in 1659, Peyrepertuse became of little strategic importance and was practically abandoned.Today visitors can make the short hike up the hill (after a long and winding drive, that is) and explore the various chambers and outposts of the still impressive structure. The Chapel of San Jordi is probably the best known architectural feature of the fort, and on sunny days, it commands sweeping views all the way to the Mediterranean in the east, and to the Pyrenees in the south.
I had the misfortune of visiting on a stormy day, however, as you'll see from the video below that I shot at the castle. Even without the views though, you can see what a grand, forbidding place it is.