Visit Cathar Country
The term "Cathar" came into centuries later; the people we now call Cathars would have called themselves "Good Men" or "Good Christians". In this entry I use the familiar term Cathar.
Cathar country refers to the regions controlled by Raymond VI of Toulouse, Raymond Roger Trencavel, and the Comte of Foix (and lesser nobles and vassals). When Pope Innocent III called for a crusade to eradicate the Cathar heretics, the crusader army overran this region, first taking the walled cities in the open plains and later the castles in the foothills of the Pyrenees.
The mountain strongholds are sometimes called "Cathar castles" but that is a bit of a misnomer; the Cathars themselves did not have armies. The castles were defended by nobles who felt honor bound to shelter Cathars (in some cases their wives, sisters, daughters or other close family members had become Cathars).
Typically the Crusader army approached a city or castle and demanded that the Lord hand over the heretics; many of the nobles refused; and then the city or castle was besieged.
The walled city of Carcassonne is a must-see if you visit this area. However, its present day appearance is not the way it looked in the 13th century. There are now three rings of curtain walls; in the 13th century there were two. Extensive restoration made Carcassonne beautiful, but much of the work was not period correct. The view of the walls from the outside is stunning. Inside, it's full of cafes and gift shops. The museum housed in the Chateau Comtal is worth a visit. Carcassonne had not been taken for 800 years; the Crusaders took it by treachery - they held Raymond Roger Trencavel prisoner when he came outside the walls to parley.
Montsegur is famous as the place where defenders of the Cathars made a last stand. It is on a pog (steep hill):
This photo shows the steepest side; there are paths that are fairly easy to climb on other sides. The defenders only place light guards to watch this steep slope, assuming no one would attack that way. The crusaders hired professional Basque climbers ascend at night, and after they gained a foothold on top of the pog, it was easy for them to take Montsegur. About 200 Cathars were burned.
There are many other places worth a visit. My third favorite was Minerve, a village in a rocky location surrounded by deep limestone valleys that initially prevented attackers from getting their trebuchets close to the town, but eventually the attackers found a foothold. There is a replica of a large trebuchet called "La mal voisin" (the bad neighbor") and a small museum. This town is refreshingly free of tourist oriented shops. People do live there.
Another relatively well-preserved site is the castle at Puivert (Eleanor of Aquitaine may have visited it).
Crusaders generally rebuilt the castles they captured; what we see now is their work, not the pre-crusade buildings.
There are points of interest in Toulouse. Many of the other famous sites are picturesque and remote castle ruins, as in this image of Queribus at sunrise.
Even if you are not a fan of guided tours, you may want to consider one if you want to include any of the remote castles in your tour. The mountain roads are winding, steep, and narrow in places; it's sometimes challenging to find the way. Parking is limited.
There seem to be more companies offering tours in this region now than in past. I had a very positive experience with James and Sophie at Cathar Country Tours. Their web site is a great source of information, including photos.