The Royal Road was an ancient highway reorganized and rebuilt by the Persian king Darius the Great (Darius I) of the first (Achaemenid) Persian Empire in the 5th century BC. Darius built the road to facilitate rapid communication on the western part of his large empire from Susa to Sardis. Mounted couriers of the Angarium were supposed to travel from Susa to Sardis in nine days; the journey took ninety days on foot. Course of the Royal Road The course of the road has been reconstructed from the writings of Herodotus, archeological research, and other historical records. It began in Sardis near the Aegean coast of Lydia, traveled east through Anatolia (crossing the Halys according to Herodotus), and passed through the Cilician Gates to the old Assyrian capital Nineveh in upper Mesopotamia, then turned south to Babylon. From near Babylon, it is believed to have split into two routes, one traveling northeast then east through Ecbatana and then along the Silk Road (via the Great Khurasan Road), the other continuing east through the future Persian capital Susa and then southeast to Persepolis in the Zagros Mountains. Of course, such long routes for travellers and tradesmen would often take months on end, and during the reign of Darius the Great numerous royal outposts (Caravanserai) were built. History of the Royal Road Because the road did not follow the shortest nor the easiest route between the most important cities of the Persian Empire, archeologists believe the westernmost sections of the road may have originally been built by the Assyrian kings, as the road plunges through the heart of their old empire. More eastern segments of the road, identifiable in present-day northern Iran, were not noted by Herodotus, whose view of Persia was that of an Ionian Greek in the West; stretches of the Royal Road across the central plateau of Iran are coincident with the major trade route known as the Silk Road. This route was used by couriers to deliver messages to the Persian capital. However, Darius I improved the existing road network into the Royal Road as it is recognized today. A later improvement by the Romans of a road bed with a hard-packed gravelled surface of 6.25 m width held within a stone curbing was found in a stretch near Gordium and connecting the parts together in a unified whole stretching some 1677 miles, primarily as a post road,