But while the Phoenicians for their commercial intercourse with other nations relied chiefly upon the sea, the great highway of nature, they neglected by no means road-building at home. They connected their great cities, Sidon and Tyre, by a coast road, which they extended in time as far as the Isthmus of Suez. They also established great commercial routes by which their merchants penetrated the interior of Europe and Asia. Caravan roads extended south to Arabia and east to Mesopotamia and Armenia, penetrating the whole Orient as far as India, and even the frontiers of China. The Phoenicians thus became the traders of antiquity, Tyre being the link between the east and the west. The Persian Empire, which under Darius stretched from east to west for a distance of 3,000 miles and comprised no less than two million square miles, with a population of seventy or eighty millions, had, with the exception of the Romans, perhaps the best system of roads known to ancient history. Indeed, it is doubtful whether without it such a vast empire, more than half as large as modern Europe, could have been held together. Each satrap, or prefect of a province, was obliged to make regular reports to the king, who was also kept informed by spies of what was taking place in every part of the empire. To aid the administration of the government, postal communication for the exclusive use of the king and his trusted servants connected the capital with the distant provinces. This postal service was, four or five centuries later, patterned after by the Romans. From Susa to Sardes led a royal road along which were erected caravansaries at certain intervals. Over this road, 1,700 miles long, the couriers of the king rode in six or seven days. Under Darius the roads of the empire were surveyed and distances marked by means of mile-stones, many of which are still found on the road which led from Ecbatana to Babylon. These roads crossed the wildest regions of that great monarchy. They connected the cities of Ionia with Sardes in Lydia, with Babylon and with the royal city of Susa; they led from Syria into Mesopotamia, from Ecbatana to Persepolis, from Armenia into Southern Persia, and thence to Bactria and India.