All the army had tents, which were pitched so as to face the east.

The horses of the cavalry were tethered and hobbled in front of the tents of their owners.
The Persians disliked encamping near to their enemy.

They preferred an interval of seven or eight miles, which they regarded as a considerable security against a surprise.

As their most important arm was the cavalry, and as it was impossible for the cavalry to unfasten and unhobble their steeds, to equip them properly, to arm themselves, and then to mount in a short space of time, when darkness and confusion reigned around, a night attack on the part of an enterprising enemy would have been most perilous to a Persian army.

Hence the precaution which they observed against its occurrence--a precaution which was seldom or never omitted where they felt any respect for their foe, and which seems to have been effective, since we do not hear of their suffering any disaster of the kind which they so greatly feared.
The Persians do not seem to have possessed any special corps of pioneers.