It appears in the official Demo for the game.
The river Trebia. 218 BC. Having sworn an oath at his father's deathbed the young Hannibal Barca, general of the mighty Carthaginians, sets out to destroy the Romans.
Having lost many men marching across the freezing Alps, Hannibal gathers the survivors and prepares to face the might of Rome. His forces are no match for the Roman legions. Hannibal must use the element of surprise if he is to have a chance. He hides his finest cavalry in the forests. And then sends his Numidian mercenary cavalry across the river to lure the Romans into the trap.
The Romans take the bait and chase the Numidians back across the freezing river.
And the scene is set for Hannibal to stage his ambush. Timing is crucial.
If his cavalry charge too soon, they'll be isolated and cut down by the Roman infantry. If they charge too late, the Carthaginian front line will buckle before the Roman advance. Hannibal also knows that he must use his elephants to create fear and terror.
Failure to use these two key units to maximum effect will spell disaster for Hannibal’s campaign against Rome.
Hannibal's army is composed of mainly of mercenaries, ranging from light cavalry to light infantry to archers. He also commands a host of Carthiginian troops, containing elephants, heavy cavalry, spearmen and skirmishers.
The Roman army is composed mainly of infantry and spearmen, with a small force of cavalry and skirmishers.
As Rome secured more control of the Italian peninsula, Carthage was establishing an empire in Spain and North Africa. It was inevitable that these two rising powers would clash violently over control of the Western Mediterranean. Carthage needed space for its trade and maritime empire, and the Romans simply saw Carthage as another threat. War was bound to come, sooner or later.
Fortunantely for the Carthiginians, they had the service of Hannibal Barca, one of the greatest generals of all time. Marching from Spain, he slipped past one Roman army and into northern Italy over the Alps, even managing to bring a contigent of elephants with his army. His bold strategy was to march at Rome and break Roman power at its heart. He also hoped to gain allies as he advanced, among people disenchanted with the domination of Rome.
After two months of campaigning he was met by a Roman force at the river Trebia. Hannibal used a feigned cavalry retreat to lure the Romans across the freezing river. True to form, and despite the winter floods, the Romans crossed the river and made straight for Hannibal's army. The few Romans who escaped were the ones who broke through the Carthiginian line as it closed arround them and then kept running!