It is difficult to understand how they posted all these troops in front of the phalanx, considering that the river ran close past the camp, especially in view of their numbers, for, as Callisthenes himself says, there were thirty thousand cavalry and thirty thousand mercenaries, and it is easy to calculate how much space was required to hold them.
For to be really useful cavalry should not be drawn up more than eight deep, and between each troop there must be a space equal in length to the front of a troop so that there may be no difficulty in wheeling and facing round.
Thus a stade will hold eight hundred horse, ten stades eight thousand, and four stades three thousand two hundred, so that eleven thousand two hundred horse would fill a space of fourteen stades.
If the whole force of thirty thousand were drawn up the cavalry alone would very nearly suffice to form three such bodies, one placed close behind the other.
Where, then, were the mercenaries posted, unless indeed they were drawn up behind the cavalry? This he tells us was not so, as they were the first to meet the Macedonian attack.
We must, then, of necessity, understand that the cavalry occupied that half of the space which was nearest the sea and the mercenaries the half nearest the hills,
and from this it is easy to reckon which was the depth of the cavalry and how far away from the camp the river must have been.
After this he tells us that on the approach of the enemy, Darius, who was half way down the line, called the mercenaries himself from the wing to come to him. It is difficult to see what he means by this.
For the mercenaries and cavalry must have been in touch just in the middle of the field, so that how, why, and where could Darius, who was actually among the mercenaries, call them to come to him?
Lastly, he says that the cavalry from the right wing advanced and attacked Alexander's cavalry, who received their charge bravely and delivering a counter charge fought stubbornly.
He forgets that there was a river between them and such a river as he has just described.
Walbank Commentary