"Nobody knows how old Petra is, but it was a thriving city when Abraham left Ur of the Chaldees, and for a full five thousand years it has had but that one entrance, through a gorge that narrows finally until only one loaded camel at a time can pass. Army after army down the centuries have tried to storm the place, and failed, so that even the invincible Alexander and the Romans had to fall back on the arts of friendship to obtain the key. We, the last invaders, came as friends, if only Grim could persuade the tyrant to believe it...