Central Asian grass lands started drying up some 4000 years ago and since then, the hordes have been moving southward and westward. China built the great wall to protect its empire. Rome was invaded by the Huns, Mangols devastated the empire of Baghdad. In the last millennium Arabs tamed these wild horsemen put the foundation of the desert kingdom. The cities became the center of learning for whole of the orient. They develop finest specimen of art and architecture Khiva, Samarkand and Bokhara are, the living examples.