Any classification of the types of human settlement in Iran must be made on a scrupulously genetic basis; and care should be taken to distinguish among the various forms arising at different periods. Several types of rural habitats were prevalent in Iran including ancient village settlements, rural habitat in heavy rainfall-areas on Caspian Sea shores, and rural habitat in the open plains. The qal'eh village is the product of a pastoral civilization that was fraught with insecurity; it reflects the settled people's need to defend themselves, and their cattle, against the repeated incursions of the nomads. A map of the urban network as it is today shows the fundamentally asymmetrical disposition of the towns within Iran, and their relationship both with natural conditions and with the density of the rural population. Cities and towns are numerous in all those regions where rainfall is sufficient for agricultural purposes. They are located in the centre, the east, and the south-east of Iran.