When the mosque cannot keep up with demographic growth, a new one must be built and a new city must be founded. This rule has provided a constant regulation of urban development through the centuries. When the population becomes too numerous the settlement does not begin to sprawl beyond the city limits, instead the original pattern is reproduced by building a new city, in a similar geomorphologic situation, presenting the same characteristics as the older one. This method has brought about the progressive development of the settlements on high and unproductive crags, which can be easily defended, thus protecting the bottom of the valley where palm groves form a long green ribbon extending over about one thousand hectares. Fruit trees grow in the shadow of the palms and under the trees, one discovers carefully tended plots of vegetables and grains.
Water is drawn from wells since no surface water is to be found.
Sunken dams of imposing size were built to preserve the humidity of the soil by intercepting the underground microflow. This makes it possible to maintain the moisture of the soil of the palm grove. The main water resource comes from the flooding of the M'zab wadi that occurs every three to ten years. The entire valley is organised in view of this event. Large sluices intercept the flow and direct it towards the tilled fields.