The gorge is brief but remarkable—none the less so for taking us to the Castle of Puilaurens. In the twelfth century, a strange, austere, anti-Catholic movement called Catharism took root in the south of France. The Church put it down so brutally that one abbot, leading a crusading army storming the city of Béziers, uttered the phrase that in a different form would eventually be embroidered on the back of many a U.S. Marine's jacket. When a soldier asked the abbot how to distinguish the Catholics from the heretics among Béziers's inhabitants, he replied: "Kill them all. God will recognize his own." In Vietnam it became, "Kill 'em all, let God sort 'em out." God had 20,000 to catalog that day, butchered in Béziers. Only 220 of them were Cathars.

Castles like (what's left of) the one high above the village of Puilaurens are scattered throughout Roussillon and Languedoc, aeries atop the highest crags. Glancing down a thousand-foot drop, I pick a couple of leaves from a mintlike plant growing inside the castle walls and rub them between my fingers to see if there's a scent. No smell but excruciating pain: stinging nettles, covered with near-invisible needles that now live happily under my skin. My wife helpfully tells me that in East Africa, where she has trekked, they have something called doctor plant that can be rubbed on to ease the pain. But she doesn't know of any in France.

One more diversion before our destination: the tiny hill town of Rennes-le-Château, which has become a point of pilgrimage among enthusiastic readers of the airport novel The Da Vinci Code. Rennes-le-Château is enigma central, a favorite of conspiracy theorists, lost-treasure seekers, Shroud of Turin dry cleaners, Holy Grail searchers, Second Coming awaiters, and pyramid-power advocates. It is France's Area 51.

Much of it has to do with a local priest named Saunière—yes, like the Louvre curator who is murdered in the opening scene of Dan Brown's book—who in the late nineteenth century supposedly became unspeakably wealthy, sharing the source only with his "housekeeper." They both died (she, finally, in 1953) without revealing the secret, and theories range from his having found a buried Templar stash to having held the Vatican's feet to the fire because he'd discovered evidence that Jesus had married Mary Magdalene. At best, Rennes-le-Château is a delightful village with sweeping views, thanks to its height, and a charming open-air restaurant in the shaded little plaza. At worst, it's a black hole that sucks in the obsessed, who go on to write books about "the mystery of Rennes-le-Château."

Like Mont-St-Michel, Carcassonne is most movingly seen from a distance. Up close, it turns out to be a jostle of theme bars, bad restaurants, T-shirt shops, and souvenir stores selling plastic suits of armor, but the long shot of the restored medieval fortress-city is just as fabulous as it looks on postcards.