It’s late July in 1209. You wake up to yet another cloudless day, the morning fresh but free of dew—it’s far too dry. In the afternoon, the blazing summer sun will turn the stone walls and cobbled lanes of your fortified town, Carcassonne, into an oven.

The ensemble toward the right is the castle within the fortified city.

In the streets, people are worried. Reports have arrived of the massacre at Béziers. Pope Innocent III had called for a crusade against the Cathars, a dualist religion spreading in the south of France. About 10,000 crusaders arrived at Béziers on July 21, 1209, asking for the surrender of the city. When the locals refused, the crusaders the next day slaughtered the population, Cathars and Catholics alike. When asked how to distinguish the Catholics from the Cathars, the knight in charge, Arnaud-Amaury, supposedly said, “Kill them all; God will recognize his…