The Persian Empire suffered a series of defeats against the Macedonian forces at Granicus (334 BC), Issus (333 BC) and Gaugamela (331 BC), and by the end of 331 BC Alexander had advanced to Babylon and Susa. A Royal Road connected Susa (the first Persian capital city in Elam) with the more eastern capitals of Persepolis and Pasargadae in Persis, and was the natural venue for Alexander's continued campaign. Meanwhile, King Darius III was building a new army at Ecbatana. Ariobarzanes was charged with preventing the Macedonian advance into Persis, and to this effect he relied heavily on the terrain Alexander needed to pass through. There were only a few possible routes through the Zagros Mountains, all of which were made more hazardous by winter's onset.