The Battle of Alesia has been credited to be the major turning point of Roman Expansion, while marking the rise of Gallic Empire under Arvernic Dynasty, in which Vercingetorix and his descendants dominate the Gallic, Germanic, and Northern Italy for more than a century.
It was said that between the need to Avenge both Crassus' and Caesar's deaths, Gaius Pompeius Magnus choose to went to conquer the East while the less experienced Cato were elected to led the concurrent reprisal against Vercingetorix. This led into disaster as Vercingetoric managed to roll the Roman Army under Cato and pressed deep into North Italy. Forcing Pompeii to hastily withdrawn from the east to relieve Rome from Gallic siege. But then the Ptolemaic Egypt, Lusitania and Numidia sensed Roman Weakness and took away Syria and Anatolia, Iberia, and Roman North Africa.
Rome as a political entity survived the ordeal, albeit being reduced to Central and South Italy, Sardinia, and Sicily, with North Italy returned to become their frontier region against the now (ironically) romanized and organized Gauls.
Many did said Caesar's fate was sealed when he was sandwiched between the besieged Forteess-Town of Alesia, and the incoming Gallic reinforcement that moves to relieve the besieged Fortress-Town. His attempt to built two layers of siege walls actually made his forces getting caught in the middle, and only a small group of Cavalry under Marc Anthony survived the battle.
Now, what if Caesar, recognizing the futility of the siege, retreat and fight the Gauls another day? Or actually made the double wall quicker and already prepared to be sandwiched?