After defeating the Gallic forces of Vercingetorix near Divio, Caesar followed his retreating army to the fortified town of Alesia. With an alleged army of some 80,000 men, Vercingetorix and his Gauls were in shock from Caesar's Germanic cavalry allies and were in no condition to meet the 60,000 Romans Legionaries on the battlefield. In the seasonal summer of 52 BC (late September by the calendar of the time), Caesar approached Alesia with the Gauls holed up inside and was well aware of his army's failures at Gergovia just a short time before.
Deciding to wisely forego a direct attack, Caesar knew that to hem the Gauls inside the fort would eventually starve them out, as food was scarce prior to the fall harvest. Caesar, in his commentaries, suggests that the Gauls had only as much as 30 day's food to feed his army, even on limited rations. He ordered the complete circumvallation of the Alesian plateau, which would not only enclose the Gauls, but keep his large army occupied during the siege. Walls, ditches and forts of various sizes stretched the entire circle for a total length of 10 miles. A wide ditch was dug out in front of the works, with a second water filled trench behind it, separating the open 3 mile field between Alesia and the Roman wall. Caesar himself gives a detailed explanation of the construction:
"he dug a trench twenty feet deep, with perpendicular sides, in such a manner that the base of this trench should extend so far as the edges were apart at the top. He raised all his other works at a distance of four hundred feet from that ditch. Having left this interval, he drew two trenches fifteen feet broad, and of the same depth; the innermost of them, being in low and level ground, he filled with water conveyed from the river. Behind these he raised a rampart and wall twelve feet high; to this he added a parapet and battlements, with large stakes cut like stags' horns, projecting from the junction of the parapet and battlements, to prevent the enemy from scaling it, and surrounded the entire work with turrets, which were eighty feet distant from one another."
Though the construction went well, as would be expected with 60,000 laborers, Vercingetorix didn't sit back and watch his army become encircled. Regular raids attempted to interrupt the Romans, but the legions, accompanied by the fearsome Germanic cavalry, outmatched the attackers and sent them scattering back to their own fort. To cut down the number of attacks, and likely escape attempts, Caesar next ordered an elaborate system of traps and additional wall defenses. He continued to explain,
"It was necessary, at one and the same time, to procure timber. Having, therefore, cut down the trunks of trees or very thick branches, and having stripped their tops of the bark, and sharpened them into a point, he drew a continued trench every where five feet deep. These stakes being sunk into this trench, and fastened firmly at the bottom, to prevent the possibility of their being torn up, had their branches only projecting from the ground. There were five rows in connection with, and intersecting each other; and whoever entered within them were likely to impale themselves on very sharp stakes. The soldiers called these "cippi." Before these, which were arranged in oblique rows in the form of a quincunx, pits three feet deep were dug, which gradually diminished in depth to the bottom. In these pits tapering stakes, of the thickness of a man's thigh; sharpened at the top and hardened in the fire, were sunk in such a manner as to project from the ground not more than four inches; at the same time for the purpose of giving them strength and stability, they were each filled with trampled clay to the height of one foot from the bottom: the rest of the pit was covered over with osiers and twigs, to conceal the deceit. Eight rows of this kind were dug, and were three feet distant from each other. They called this a lily from its resemblance to that flower. Stakes a foot long, with iron hooks attached to them, were entirely sunk in the ground before these, and were planted in every place at small intervals; these they called spurs"
Now as the Romans approached completion of the enclosure which took them only 3 weeks, Vercingetorix ordered some of cavalry to attempt a break out under cover of darkness. With some gaps still in unfinished fortifications, some cavalry is able to escape to nearby tribes and call upon them to help lift the siege. Caesar, however, because of desertions and captured cavalry is well aware of the plan and realizes that his enclosure of Alesia won't help against a relief army. In one of the most brilliant siege tactics in the history of warfare, and a testament to the skill of Roman engineering, Caesar ordered a second wall to be built on the outside of the first. This wall, nearly identical to the first in construction and type, extended as much as 15 miles around the inner wall and left enough of a gap in between to fortify the entire Roman army. Protected from sally attempts by Vercingetorix and the relief army that was sure to come, the Romans waited for the relief force.
The cavalry that escaped did manage to rally support for Vercingetorix, and a massive army, especially by 'barbarian' standards was raised. According to Caesar, nearly 250,000 Gauls came in support of their besieged 'King', but most modern historians estimate this number at a less daunting number of around 100,000. Even so, this force marched from the territory of the Aedui to crush the Romans between two forces larger than that of their target. Inside Alesia, however, conditions were terrible, with an estimated 180,000 people (including non-combatant women and children) running out of food and supplies. The resident tribe, the Mandubii, sent out their women and children as a delay tactic, hoping that Caesar would let them pass to safety, but the Romans let them starve at the inner Roman wall to show the Gauls the error of their ways, and to prevent foul play.
By the time the relief force arrived, Vercingetorix and his army were in dire straights, with many of his men likely on the verge of surrender. The relief force arrived just in time however, heartening the resolve of the besieged and setting the stage for the battle that would make or break Caesar's fortunes in Gaul. In late Spetember, 52 BC, the battle began with a charge from the Gauls on the exterior of the Roman fortifications. A hard fought engagement from noon to sunset ensued, with neither side having a clear advantage. Both Romans and Gauls fought with equal valor and inspired by the fight, Vercingetorix led his men out of Alesia towards the inner Roman wall. Unable to penetrate the defenses, he was not able to lend support to his countrymen, and eventually Caesar's Germanic cavalry turned the Gallic flank and sent them back to their camps.
The next day, the Gauls outside the Roman works prepared ladders, hooks and equipment for scaling the walls. Around midnight, they put their equipment to use, launching an all out night attack. The initial battle went very well for the Gauls, and many Roman defenders were killed. At the hardest fought points in the defenses, Marcus Antonius and Caius Trebonius saved the day for the Romans, by abandoning those posts that were free from attack and moving these reinforcements into the heaviest action. Eventually, Roman artillery prevailed and the outer Gauls were forced to retreat. On the inner wall, meanwhile, Vercingetorix had launched a simultaneous attack, but because of the Roman trenches, which the Gauls had to fill, the attack was delayed too long. As dawn began to break, Vercingetorix was forced to retreat as well, since his compatriots on the outer wall had to retire.
The next day, around October 2, would prove to be the final battle for Alesia. Sometime around mid day, a force of 60,000 Gauls under Vergasillaunus discovered a weakness in the Roman lines. Because of natural obstructions, there were areas in the defense works where walls simply couldn't be built. The Gauls on both the outside and inside launched a simultaneous attack on all quarters of the Roman works. Vergasillaunus pressed hard on the weak part of the wall, while Vercingetorix occupied the Romans all over the inner wall. The Gauls pushed the Romans back all over and the battle was on the brink of disaster for Caesar. Only the vaunted Roman discipline seems to have prevented a complete rout. Caesar himself, rode all over along the Roman lines lending support and encouragement to his men to hold the lines. Reserves were moved wherever the situation seemed the most dire, and the orders were simply to hold their positions. Labienus was sent to relieve 2 legions of defenders against the large outer Gallic army of 60,000, with only 6 cohorts of 3,000 men. Overall, the Romans may have been outnumbered as many as 6 to 1.
Pressure was mounting all over and Caesar was forced to lead an assault on the inner wall attackers, driving them back and temporarily granting a reprieve. Labienus, meanwhile was in terrible trouble and reported that his lines were about to break. Unable to hold the defense, he would have to launch an attack in order to attempt driving the Gauls back, rather than stand back and wait for the slaughter to come. Caesar rode hard to aid Labienus and took a terrible risk in order to inspire his men. With 13 cohorts, Caesar left the relative safety of the walls and rode outside to attack the Gauls from the rear. Inspired by the sight of Caesar fighting on the outside, the Romans under Labienus launched a full with brilliant success. Sandwiched between Caesar's army at the rear and by Labienus in the front, the Gauls began to buckle and soon fell into an all out retreat. The battle that was once very close to the possible end of Caesar, turned into an all out rout and the Gauls outside the Roman walls were slaughtered. Caesar commented that if not for the complete exhaustion of his men, he might've destroyed the entire Gallic army. Even so, by the end of the action, the Germanic cavalry would virtually wipe out the retreating Gauls, leaving only Vercingetorix on the inside.
Forced back into Alesia after the defeat of his relief force, with no hope of additional reinforcements, and only with the starving remnants of his own army, Vercingetorix was forced to surrender. Caesar sat at the head of his lines and waited for the approach of the Gallic chieftans. Vercingetorix and his fellow leaders laid down their arms and surrendered quietly, where he was eventually led away to Rome. There he would sit and rot in a Roman prison for 5 miserable years, awaiting the day when Caesar could have his triumph, to be followed by the ritual execution of the enemy leader. As a reward Caesar's men each received one Gallic slave in addition to monetary spoils of war, but the war wasn't over just yet.