Plutarch, Aemilius Paulus
Bernadotte Perrin, Ed.

16. Here, then, Nasica passed the night; but to Perseus, who did not infer what was going on because he saw Aemilius remaining quietly in his position, there came a Cretan deserter who had run away on the march, bringing him news of the circuit which the Romans had taken. Though Perseus was confounded at this, he did not move his camp, but sent out ten thousand foreign mercenaries and two thousand Macedonians under Milo, with orders to make haste and occupy the passes. [2] These men, according to Polybius, ¹ were still asleep when the Romans fell upon them; but Nasica says that a sharp and perilous conflict took place for possession of the heights, and that he himself slew a Thracian mercenary, who engaged him, by striking him through the breast with his javelin, and that after the enemy had been driven away, and while Milo was flying most disgracefully without his armour or his cloak, he followed after them without danger, and brought his army with him down into the plain.

[3] After this disaster, Perseus hastily broke camp and retired; he had become exceedingly fearful, and his hopes were shattered. But nevertheless he was under the necessity of standing his ground there in front of Pydna and risking a battle, or else of scattering his army about among the cities and so awaiting the issue of the war, which, now that it had once made its way into his country, could not be driven out without much bloodshed and slaughter. [4] In the number of his men, then, he was superior where he was, and they would fight with great ardour in defence of their wives and children, and with their king beholding all their actions and risking life in their behalf. With such arguments his friends encouraged Perseus. So he pitched a camp and arranged his forces for battle, examining the field and distributing his commands, purposing to confront the Romans as soon as they came up. [5] The place afforded a plain for his phalanx, which required firm standing and smooth ground, and there were hills succeeding one another continuously, which gave his skirmishers and light-armed troops opportunity for retreat and flank attack. Moreover, through the middle of it ran the rivers Aeson and Leucus, which were not very deep at that time (for it was the latter end of summer), but were likely, nevertheless, to give the Romans considerable trouble.