Perseus saw Aemilius remaining on his ground, and had no suspicion of the truth, when a Cretan deserter, who had abandoned the Romans on the line of march, informed him of their turning movement.
The king was much disturbed; but, without moving his army, placed ten thousand foreign mercenaries and two thousand Macedonians under the command of Milo and sent him off with orders to make haste and occupy the heights.
Polybius tells us that Romans surprised this force while still asleep, but Nasica affirms that there was a sharp struggle on the heights.
Walbank Commentary