Siege of Alesia
Alesia was an oppidum (fortified settlement) on a lofty hill, with two rivers on two different sides. Due to such strong defensive features, Caesar decided on a siege to force surrender by starvation. Considering that about 80,000 men were garrisoned in Alesia, together with the local civilian population, this would not have taken long. To guarantee a perfect blockade, Caesar ordered the construction of an encircling set of fortifications, a contravallation, around Alesia. It was eleven Roman miles long (16 km or 10 modern miles, each Roman mile equal to 1,000 paces), and had 23 redoubts (towers). While work was in progress, the Gauls carried out cavalry sallies to disrupt the construction. Caesar placed the legions in front of the camp in case of a sortie by the enemy infantry and got his Germanic allies to pursue the Gallic cavalry.
Vercingetorix sent messengers around Gaul to rally the tribes to war and come to Alesia. When Caesar heard of this from deserters and captives, he dug a trench twenty pedes (6 metres, 19 modern feet) with perpendicular sides and built all the other works four hundred stades (probably 592 m, 1943 feet) away from that trench. The object of placing this trench so far away from the rest of the works was, as Caesar explained, that the manning of the entrenchment was not easy and, thus, this distance was a protection against surprise enemy advances at night or against javelins or other missiles being thrown at the Roman troops who were building the works during the day. Between this advance trench and the entrenchment, he dug two more trenches 15 pedes (4.45 m, 14.6 ft) wide and deep. He filled the inner one, where the ground was level with the plain or sank below it, with water from the river. Behind the three trenches he built a rampart riveted with palisades 12 pedes high (3.57 m, 11.7 ft). On top of this he built battlements (parapets with squared openings for shooting through) and breastwork (wooden screens at breast height to protect the defenders) with large horizontal pointed stakes projecting from the joints of the screens to prevent the enemy from scaling it. All round the works he set turrets at intervals of 80 pedes (24 m, 78 ft).

Some of the Roman soldiers had to go a considerable distance to get the timber for the construction of the works and grain to feed the troops. This reduced the number of troops at the Roman works. The Gauls made sorties with large forces to attack the works. Therefore, Caesar added further structures to the works to make them defensible by the reduced number of troops. Cut tree trunks were sharpened to create stakes. They were fastened at the bottom and sunk into a five pedes deep trench (1.5 m, 4.9 ft) with the boughs protruding from the ground. They were tied in rows of five so that they could not be pulled up without the attackers being impaled by the sharp stakes. Pits three pedes (0.9 m, 2.9 ft) deep which sloped inwards slightly to the bottom were dug in front of the stakes. They were placed in five intersected rows in the shape of a quincunx (an arrangement of five objects with four at the corners and the fifth at the centre). Tapering stakes, the thickness of a man's thigh, were sharpened at the top, hardened with fire, and sunk in the pits. They protruded from the bottom of the pit to a height of four fingers. Earth was pressed hard to a height of one foot from the bottom of the pit to make the stakes firm. The rest of the pit was covered with twigs and broken-off tree branches to hide the trap. Eight rows of this kind were placed three pedes (0.9 m, 2.9 ft). In front of these, one pes (0.3 m, 0.97 ft) stakes with iron hooks were sunk into the ground and scattered close to each other all over the field.
To prepare for the arrival of the Gallic relief forces, Caesar constructed an outer fortification (a circumvallation) with the same specifications but facing the opposite way as protection against the external attack by this relief force. It followed the most favourable ground and formed a circuit of 14 Roman miles (20.7 km, 12.86 modern miles).
The food supply of the population of Alesia and the 80,000 soldiers it hosted could not last long. Vercingetorix ordered all the grain to be brought to him and rationed it. The Gauls held a council, and it was decided that the old and the sick should leave the town. The inhabitants of the town also sent out their wives and children to save food for the fighters, hoping that Caesar would take them as captives and feed them. However, Caesar forbade their being admitted to his fortification, and Vercingetorix left his people out between the fortifications to starve to death.

Battle: Meanwhile, the Gallic relief force arrived and encamped on a hill one mile from the Roman fortification. The next day the Gauls encamped near the town. They then attacked the outer Roman fortification. The besieged Gauls simultaneously attacked the inner Roman fortification. However, this combined attack was unsuccessful. The next day the Gauls attacked at night. Marc Antony and Caius Trebonius brought in troops from the remotest forts in support of their comrades. At the first light of day, the Gallic relief forces, fearing being surrounded by a Roman sally, withdrew. The advance of the besieged Gauls, led by Vercingetorix, was delayed by having to fill trenches dug by the Romans. On hearing of the retreat of their comrades the besieged Gauls returned to the town.
The Gauls spotted a weakness in the Roman fortification. The north side of a hill could not be included in the Roman works and they placed a camp with two legions on steep and disadvantageous ground (this is indicated by a circle in the figure). Thus, the Gauls selected 60,000 men and appointed Vercassivellaunus, a near relative of Vercingetorix, to lead the attack on that spot. They marched there before dawn and launched the attack at noon. Vercingetorix made a sally and attacked any part of the inner fortification which seemed weak. Caesar sent Labienus to support the defense of the weak area with six cohorts of cavalry. He sent Brutus with six cohorts of cavalry and then Caius Fabius with a further seven cohorts of cavalry to defend the inner fortification. Finally, leading fresh troops, he joined in. The attack was repelled. Caesar then marched to the assistance of Labienus, drafting four cohorts and ordering part of the cavalry to follow him and part of it to leave the outer fortification and attack the Gallic relief force from the rear. Labienus was on the verge of collapse and informed Caesar of his decision of making a sally as he had been instructed. Caesar hastened. His arrival galvanised the Roman troops, who "lay aside their javelins [and] carr[ied] on the engagement with their swords." The Roman cavalry was suddenly seen at the rear of the Gauls, the Roman troops advanced rapidly and the Gauls fled. They were intercepted by the cavalry and slaughtered. The besieged Gauls were pulled back from the fortification. They fled their camps and Caesar commented that "had not the soldiers been wearied by sending frequent reinforcements, and the labour of the entire day, all the enemy's forces could have been destroyed". At midnight the Roman cavalry was sent to pursue them. Many were killed and many fled to the lands they came from.
After multiple attacks, the Gauls realized that they could not overcome the truly impressive Roman siege works. At this point, it became clear that the Romans would be able to outlast the defenders, and that the revolt was doomed. The relieving army melted away.
The next day Vercingetorix convened the Gallic council and proposed that he should be killed or surrendered alive to appease the Romans. Caesar ordered the Gauls to surrender their weapons and deliver their chieftains. The chieftains were brought before him and Vercingetorix was surrendered. Captives were given to the Roman soldiers as part of the spoils of war apart from the Aedui and Arverni, whom he hoped to win over.

Aftermath: With the revolt crushed, Caesar set his legions to winter across the lands of the defeated tribes to prevent further rebellion. Troops were also sent to the Remi, who had been steadfast allies to the Romans throughout the campaign. But resistance was not entirely over: southwest Gaul had not yet been pacified.

Alesia proved to be the end of generalized and organized resistance against Caesar's invasion of Gaul and effectively marked the end of the Gallic Wars. In the next year (50 BC) there were mopping-up operations. During the Roman civil wars Gallia was essentially left on its own.