Though it is one of the "Five Sons of Carcassonne," Puilaurens Castle is not one of the better known Cathar fortresses--like Queribus or Montsegur. That is probably because nothing too historic ever happened here, and the mountaintop on which it perches is a mere 700 meters high, so not quite as dramatic as some of the others.
Puilaurens is definitely worth a visit, though, since it is among the best preserved of the bleak Cathar castles in Languedoc, the hike is relatively easy, and it is easily accessible from the main D117 route that runs through Cathar country.
The site was originally fortified by the Visigoths right around the time of the fall of the Roman Empire. It's next date with history came in teh early 13th century during the Albigensian Crusade against the Cathars when Chabert de Barbaira turned it over to teh French forces in return for his freedom. From then, the fort played an important part in France's border defences until it, like the rest of the keeps in the area, became obsolete with the southern adjustment of the border created in 1659 with the signing of the Treaty of the Pyrenees, after which it fell into disuse and eventual ruin.
Today, the castle has a few interesting feature like the chicanes (low walls that are staggered so that you have to zig and zag in order enter the castle) leading up to the barbican, Dame Blanche's Tower, and my personal favorite, some pretty intact latrines. The view from the walls, which you are allowed to climb at certain points, also displays some pretty spectacular vistas of the surrounding mountains and forests.
As you'll see from the video below, the day I visited was an Easter Sunday of wind, rain, and fog, but it just made the experience a little more...atmospheric.