The centre and periphery were kept separate by mountains and deserts. The Caucasus, Himalaya, Karakum Desert, and Gobi Desert formed barriers that the steppe horsemen could only cross with difficulty. The city dwellers were more advanced in civilisation but they could do little militarily to defend against the mounted hordes of the steppe. Because the lowlands did not have enough open grasslands to support a large horsebound force the nomads who conquered states in China, India, and the Middle East were soon forced to adapt to the local societies.