The reasons that led the Nabataeans to settle in such an inhospitable environment are not obvious: the main ancient trade routes did not pass through it precisely, entry into the city was difficult, communications to the west and therefore towards the Mediterranean – were very difficult and the water resources were modest. In its origins at least, Petra therefore did not develop thanks to the facilities it offered to the caravans of merchants but, as the Greek historian Diodorus of Sicily recounts, because of its role as a refuge: the accesses, few and narrow, could be watched by a small number of men who announced the attacks step by step by signals of fire and by rapid messengers. From a strategic point of view, this made it possible to concentrate the troops in a single place, thus increasing their effectiveness tenfold. Moreover, in the Nabataean period, Petra was not surrounded by a continuous rampart, it was protected by a series of bastions, sometimes flanked by walls, built on the high points of the city. The location at the bottom of a basin finally made it possible to collect runoff water by simple gravity, by means of a dense network of channels carved into the rock leading to more than 200 cisterns and basins of all sizes.