The Great Desert lay between Calormen and the northern country of Archenland. It could be crossed on horseback in the span of a night and a day. The desert provided some protection from Calormen invasion since the springs at the great oasis would not have been sufficient to enable an army of any size to cross it. A secret and well-watered valley that led into Archenland did exist, though its existence was unknown to the Calormenes which forced them to rely on surprise attacks by relatively small cavalry forces. As no one from any state lived or operated there,it was, technically, an unruled state of its own.
