Fort Yarhuna lies away to the south, on the edge of the big desert. It was there, or something like it, in the days when the Sahara was still grassland; in more modern times it saw long-range patrols of Alexander the Great’s mercenaries from fair Cyrene across the sandhills eastward, and it received the battered remnants of Hannibal’s regiments after Zama. It was garrisoned by Roman legionaries before the Vandals swept into it from the west, or Arab riders from the Great Sand Sea brought the first camels and planted the date-palms in the little village beneath its walls