You can explore details of Castèl de Menerba, a key location in Trebuchets in the Garden (Book II), on the excellent Cathar Castles site.
The site also has an excellent new page on little-known Castèl de Les Cassés Vielh.
The pictures of Minerve from Cathar Castles (like the one shown on the left) give you a good sense of the natural defenses that the mayor and citizens of Minerve thought would protect them from the French invaders.
When the “crusaders” arrive at Minerve in the chapter “Cher Malvoisine,”:
At the cross-over moment in the morning, the squeak and chirp of the birds died away, replaced by the persistent fiddling of crickets as the sun heated the ground and the shimmer rose. The bird cries ended earlier and earlier each day: songbirds fled the plateau with the arrival of the men who laid siege on Minerve. The crusaders who arrived late had to work hard to find material for shelter, reduced to weaving mats of brush and thorny twigs to create shade in the sweltering June sun. Only the locusts, scorpions, and field crickets were content to ignore the thousands of invading crusaders.
The silky skies held no promises for the sweltering crusaders, not a puff of cloud or prayer’s chance of rain. The winds from the north remained too faint to ease the sweat from one’s brow, and the occasional gusts blasting across the plateau only stirred up the dust and drove it like powder into their tents and beds, under chainmail so it clung to the folds of their skin, and into their food supplies so every meal was a chore, with sandy dust in every bite.