‘Surra-man-raā’, says Mas'ūdī, ‘is the last the great cities founded in Islam.’ Seven in all, till the time of Mas'ūdī, these were according to him: Baṣra, founded in 17/638–9; kūfa, founded in 17/638–9; Fusṭāṭ, founded in 20/640–1; Ramla and Wāsiṭ, founded in 13–14/634–6; Baghdād, founded in 145/762–3; and surra-man-ra'ā, founded in 221/834–5. The one common feature was that, except for Ramla, these towns were all garrison centres. Built on the edge of the desert, the natural refuge for the predominantly nomadic Arab invading armies. the first five cities maintained all along an Arab-Islamic tradition. Emerging almost at the same time (save for Ramla) and marking at the respective moments of their emergence, historical landmarks in the rapid expansion of the Islamic empire in its first stages of growth, these five cities were populated on the whole by Muslim Arabs, who were to safeguard the conquests and gradually absorb rather than be absorbed by other elements. These cities were thus able to maintain that Arab-Islamic tradition which, challenged later by the heterogeneous cultural heritage of what became the Islamic World, proved strong enough to make and keep that world Islamic, though not wholly Arbic. This result was the fulfilment of the policies of the Caliph 'Umar I, under whose reign the major Islamic conquests were made and who, for direct strategic reasons, advised his generals to encamp their troops on the borders of the desert Arab lands and of the newly acquired territories.