Location
Darius chose a flat, open plain where he could deploy his numerically superior forces, not wanting to be caught in a narrow battlefield as he had been at Issus two years earlier, where he was unable to properly deploy his huge army. Darius also had his soldiers flatten the terrain prior to the battle, so as to give his 200 war-chariots optimal conditions to operate in. However this did not matter. On the ground were few hills and no bodies of water that Alexander could use for protection, and in the autumn the weather was dry and mild. The most commonly accepted opinion about the location is (36°22′N 43°15′E / 36.36°N 43.25°E / 36.36; 43.25), east of Mosul in modern-day northern Iraq – suggested by Sir Aurel Stein in 1938 (see his Limes Report, pp. 127–1)
Read more about this topic: Battle Of Gaugamela