A higher-res version is available if you view it on Youtube.
You can watch as the Warband advances with a skirmish line out front. The skirmish line advances to just short of the Roman legions, and mucks up their line with some surprisingly effective shooting.
The Romans redress their line and advance, but one unit of Legionaries is left out front, and the warbands capitalize and destroy it.
The Romans finally manage to come to grips with part of the main force of Celt infantry, but a rogue unit of Celt skirmishers manages to get between the Romans and the rest of the Celts and block them.
Meanwhile, on the peripheries, the Celt chariots and cavalry wipe out enough of the Roman sidelines to pass the Roman break point, and the Celts win the day.