Most important battle of the Gallic Wars - Romans reached the important town of Alesia that was built on the hill. The defenders consisted from different Gallic tribes were led by Vercingetorix, who was confident that the town can’t be taken by force - the terrain and fortifications were on their side. Julius Caeser, who led the Roman legions, probably shared that view and decided not to strike on the town, but to start the siege. But to make sure that the ring around Alesia will be tight he decided to build fortifications around the town, or to be exact two sets of fortifications - one to provide defenders from escaping from Alesia and another to provide any relief forces that would try to reinforce the defenders.
It was huge logistic and engineering challenge, since the fortifications around Alesia were 18 kilometres long and 4 meters high. But this plan worked and soon the defenders were more and more desperate. They managed to make use of the only weak point of the fortifications (over the rivers) and passed the message to other tribes requesting help. Gauls managed to gather large army of about 100,000 (comparing to only about 50,000 Romans) and made an attempt to break the siege. At the same time the defenders made the push to drag the Romans into battle.
The attack failed to reach the target - Julius Ceasar managed to drive the cavalry units around and attack the relief forces from behind. The attack has broken and the defenders had no other option than surrender. Vercingetorix was executed.
Life of Julius Ceasar from the age of 18 until his assassination.