For in the first place, as the battle was between three armies, it is evident that the appearance and the movements of the forces marshalled against each other must have been in the highest degree strange and unusual.
Again, it must have been to all present, and still is to us, a matter of doubt whether the Celts, with the enemy advancing on them from both sides, were more dangerously situated,
or, on the contrary, more effectively, since at one and the same time they were fighting against both their enemies and were protecting themselves in the rear from both, while, above all, they were absolutely cut off from retreat or any prospect of escape in the case of defeat,
this being the peculiarity of this two-faced formation.
The Romans, however, were on the one hand encouraged by having caught the enemy between their two armies, but on the other they were terrified by the fine order of the Celtic host
and the dreadful din, for there were innumerable horn-blowers and trumpeters, and, as the whole army were shouting their war-cries at the same time, there was such a tumult of sound that it seemed that not only the trumpets and the soldiers but all the country round had got a voice and caught up the cry.
Very terrifying too were the appearance and the gestures of the naked warriors in front,
all in the prime of life, and finely built men, and all in the leading companies richly adorned with gold torques and armlets.
The sight of them indeed dismayed the Romans, but at the same time the prospect of winning such spoils made them twice as keen for the fight.
Walbank Commentary