In 225 BC a key battle in the development of Cisalpine Gaul and probably the development of Rome, was fought at Telamon.
Rome and the tribes of Cisalpine Gaul have been in relatively peaceful relations for a period. So peaceful that the Boii of the territory had been doing much of the actual work preventing Transalpine Gauls and other Celts from invading Italy.
However, in 232 B.C., during the consulship of Marcus Aemilius Lepidus, the tribune Caius Flaminius carried a bill to distribute to the Roman citizens the ager Gallicus, a territory in Cisalpine Gaul known as Picenum, from which they had ejected the Senones after they conquered them. This bill prompted many of the Gauls and especially the Boii, whose territory bordered on that of the ager Gallicus to take action against Rome with the conviction that now the Romans no longer made war on them for the sake of supremacy and sovereignty, but desired their total expulsion and extermination.
Their response was to hire a substantial army of Gaesatae mercenaries from across the Alps to aid them in a coming fight with the Romans. They urged and incited their kings Concolitanus and Aneroëstus to make war on Rome, offering them for the present a large sum in gold, and as for to the future, pointing out to them the great prosperity of the Romans, and the vast wealth that would be theirs if they were victorious.
The Romans grew fearful of the Celtic army approaching. The Romans, the moment they heard that the Gauls had crossed the Alps, sent Lucius Aemilius, their Consul, with his army to Ariminum to await there the attack of the enemy. They also sent of their Praetors to Etruria and their other Consul, Gaius Atilius, had already gone to Sardinia with his legions. There was great and general alarm in Rome, as they thought they were in imminent and serious peril, and this indeed was but natural, as the terror the old invasion had inspired still dwelt in their minds.
Etruria was overrun, and the Gauls won a victory at Faesulae. The Gauls withdrew with their loot for the time, deposited it in a friendly tribe's country, then returned. The Roman army of Aemilius Papus followed them as they marched with all their loot. By more good luck than strategy Atilius Regulus arrived in front of the Gallic army. In this situation, both consuls would be able to attack the Celts from different sides.
Atilius Regulus positioned his cavalry on a hill. The Gauls skirmished with them, but only then realized they had been ambushed. Regulus was killed, despite relief from Aemilius Papus, his head taken as a trophy, but the Romans held the hill. In response to the enemy now facing them on three sides, and further blocked in by the coast, the Gauls quickly drew up into two opposing lines to face the Roman ambush.
So fine was their order and their speed that the Romans were apparently taken to fear. The battle would be long, and extremely bloody, but in the end, the Romans were victorious through superior position and numbers, as well as the ability to crush the Gauls into eachother as the noose tightened, making their weapons less useful in close quarters than the Romans' own shorter swords. The Gallic leader Aneroestes escaped and committed suicide with his followers. Concolitanus, the other leader of the Gauls, was taken prisoner by the Romans.
If the Romans had not been victorious over the Gauls, it is likely Concolitanus and Aneroestes would've continued on toward Rome, such as the plan had intended. What happened from there would still be left to imagination, but the loss of a substantial Roman force would be a major setback to attempts to defend the city, possibly allowing it to be sacked again by Gauls.

Battle: As the Gauls you will have to fight for your life against the two Roman armies approaching from different sides.