As usual it's been a while, so let's go right into it. Persia is fielding an almost full army (eleven units) of mainly spearmen and cavalry. At the left they are screened by hills, next to which they place their camp. Darius deploys his army as a single body, takabara at the centre with his cavalry in the front, flanked by light units of horsemen and skirmishers.
Alexander's army is divided in three wings. At the far left a single ballista is protected by light prodromoi, at the right he places his companions and the centre is dominated by five regiments of pikemen, supported by slingers, all ready to exit the woods.
Darius is moving forward fast and the armies are quickly forming for the clash. At the far edge of the battlefield Alexander's prodromoi are drawing a unit of persian infantry supported by skirmishers. The persians have spread their cavalry keeping their main infantry in reserve. The macedonian cavalry is ready for the charge on the right, while the phalanxes are still forming at the centre.
The armies are drawing near, soldiers slowly locking their eyes on individual men they will fight. At the persian left there will be a cavalry battle, Darius having the upper hand with his bactrians facing the outnumbered companions. At the centre, the phalanxes seem like an iron porcupine, making the persian spearmen nervous.
A great beginning for Darius as his greek hoplite mercenaries, mixed with persian cavalry, stop the companions in their tracks and rout them, further weakening Alexander's right flank. His phalanxes are struggling to form and advance.
The macedonian general orders his right flank to redeploy further back, the two remaining companion units and the slingers behind them. The persians have to decide whether to keep pressing that flank or deal with the pikemen, who are still not properly lined up.
The battle is turning into a persian triumph and Darius is rubbing his hands. As the phalanx broke into fragments, a regiment got flanked from both sides and after a continuous shower of persian arrows, the pikemen fled through their comrades and dispersed into the woods behind. The macedonians are now hard-pressed against the forest. At the far left, the prodromoi ran away from the persian takabara.
Overview of the battle: the greeks are pressed from every side while Darius is not letting them catch their breath. Cold sweat is trickling down Alexander's spine as he realises this day might be lost.
The macedonians make a last attempt and move forward, their right flank looking barely threatening, the weakened phalanxes forming again. The prodromoi are getting back to the fight, facing the takabara at the persian right.
The persian left forms quickly as bactrian horsemen and Darius' satraps with their finest guards rush forward for their great king's glory. They easily cut down the greek slingers and continue to push the companions back.
Alexander has seen enough. He orders the retreat and his officers carry the order throughout the army. Darius' men start celebrating while on the right his zealous bactrians chase down the battered prodromoi who are slain by the dozens.
As the dust settles, the men sit where they stand, desperate for some rest. The vultures are gathering above the field and the Persian king of kings surveys today's bloody work. In the distance, among the defeated Macedonians, he thinks he spots a figure atop a black stallion, who locks his eyes on Darius for a moment and then turns away disgusted. Alexander takes out his humiliation on his horse, Bucephalus, and starts galloping hard, away from the battlefield.
This was a tense situation but it seems the battle was always under the control of the persian army. The greeks made a big mistake placing all those pikes inside or in front of the forest. The persians moved aggressively and attacked the pikes just in front of the bad-going woods. Also at the greek right, the persians flanked the enemy quickly, proving they constantly had the initiative, and pushing the greek cav back into a defensive position. The greek light horse on the other side kept some persians busy but not hard enough or long enough. Although two of the greek casualties happened at the flank, the battle was pretty much decided at the centre where the pike superiority was nullified by the forest and the speed of the persian army.