How much wetter was the climate back then? Nine miles of fortifications can’t be patrolled, much less defended, by the manpower available in a town of 500. Presumably there was a much larger population outside the town proper.
After the drying out and cooling of the Younger Dryas, there was still more foliage than now. Settlements in Arabia relied on natural water sources, so, oases, and while the wells endured the population did as well. The advantage to the settlements wat the low population density in surrounding areas, so, any attacks on the walled settlements were probably ineffective and outnumbered by the defenders.
https://whc.unesco.org/en/tentativelists/6575/