Siege of Alesia
Background: In 58 BC, following his first consulship in 59 BC, Julius Caesar engineered his own appointment as proconsul (governor) of three Roman provinces by the First Triumvirate. These were Cisalpine Gaul (northern Italy), Illyricum (on the eastern coast of the Adriatic Sea) and Gallia Narbonensis (in southeastern France and the rest of France's Mediterranean coast). Although the proconsular term of office was meant to be one year, Caesar's governorship was for an unprecedented five years. He also had the command of four legions.
Caesar engaged in the Gallic Wars (58–50 BC), which led to his conquest of Gaul beyond Gallia Narbonensis. When the Helvetii, a federation of tribes from what is now Switzerland, planned a migration to the Atlantic coast through Gaul, Caesar went to Geneva and forbade the Helvetii to move into Gaul. While he went to Gallia Cisalpina to collect three other legions, the Helvetii attacked the territories of the Aedui, Ambarri, and Allobroges, three Gallic tribes, which called for Caesar's help. Caesar and his Gallic allies defeated the Helvetii. The Gallic tribes then asked for Caesar to intervene against an invasion by the Suebi, a Germanic tribe. Caesar defeated the Suebi and, to demonstrate Roman power,  crossed the Rhine in 56 BC.  In 57 BC he intervened in intra-Gallic conflicts and marched on the Belgae of northern Gaul. From then on he conquered the Gallic peoples one by one. His successes in Gaul brought Caesar political prestige in Rome and great wealth through the spoils of wars and the sale of war captives as slaves.
Gallic existential concerns came to a head in 52 BC and caused the widespread revolt the Romans had long feared. The campaigns of 53 BC had been particularly harsh, and the Gauls feared for their prosperity. Previously, they had not been united, which had made them easy to conquer. But this changed in 53 BC, when Caesar announced that Gaul was now being treated as a Roman province, subject to Roman laws and religion. This was a subject of immense concern for the Gauls, who feared the Romans would destroy the Gallic holy land, which the Carnutes watched over. Each year the druids met there to mediate between the tribes on the lands considered the center of Gaul. A threat to their sacred lands was an issue that finally united the Gauls. Over the winter the charismatic king of the Arverni tribe, Vercingetorix, assembled an unprecedented grand coalition of Gauls.

Prelude: Caesar was still in Rome when news of the revolt reached him. He rushed north in attempt to prevent the revolt from spreading, heading first to Provence to see to its defense, and then to Agedincum to counter the Gallic forces. Caesar took a winding route to the Gallic army to capture several oppidum for supplies. Vercingetorix was forced to withdraw from his siege of the Boii (allied to Rome) capital of Gorgobina. However, it was still winter, and Vercingetorix realised the reason Caesar had detoured was that the Romans were low on supplies. Thus, Vercingetorix set out a strategy to starve the Romans. He avoided attacking the Romans outright, and instead raided foraging parties and supply trains. Vercingetorix abandoned a great many oppidum, seeking to only defend the strongest, and to ensure the others and their supplies could not fall into Roman hands. Once again, Caesar's hand was forced by a lack of supplies, and he besieged the oppidum of Avaricum where Vercingetorix had pulled back to.
Vercingetorix had originally been opposed to defending Avaricum, but the Bituriges Cubi had persuaded him otherwise. The Gallic army was camped outside the settlement. Even while defending, Vercingetorix wished to abandon the siege and outrun the Romans. But the warriors of Avaricum were unwilling to leave it. Upon Caesar's arrival, he promptly began construction of a defensive fortification. The Gauls continuously harassed the Romans and their foraging parties while they built their camp, and attempted to burn it down. But not even the fierce winter weather could stop the Romans, and a very sturdy camp was built in just 25 days. Siege engines were built, and Caesar waited for an opportunity to attack the heavily fortified oppidum. He chose to attack during a rainstorm, where the sentries were distracted. Siege towers were used to assault the fort, and artillery battered the walls. Eventually, the artillery broke a hole in the wall, and the Gauls were unable to stop the Romans from taking the settlement. The Romans then looted and raped the settlement; Caesar took no prisoners and claimed the Romans slew 40,000. That the Gallic coalition did not fall apart after this defeat is a testament to the leadership of Vercingetorix. Despite this setback, the Aedui were willing to revolt and join the coalition. This was yet another setback to Caesar's supply lines, as he could no longer get supplies through the Aedui (though the taking of Avaricum had supplied the army for the moment).

Vercingetorix now withdrew to Gergovia, the capital of his own tribe, which he was eager to defend. Caesar arrived as the weather warmed and fodder finally became available which somewhat eased supply issues. As usual, Caesar promptly set to building a fortification for the Romans. Caesar then set about capturing territory closer to the oppidum. What happened in the ensuing Battle of Gergovia remains somewhat unclear. Caesar claimed that he had just ordered his men to take a hill near to the oppidum, and that he then sounded a retreat. But no such retreat occurred and the Romans directly assaulted the settlement. Gilliver finds it likely that Caesar did not actually sound a retreat and that it was his plan all along to directly assault the settlement.  Caesar's dubious claim is likely to distance himself from the ensuing and overwhelming failure of the Romans. The Roman assault ended in clear defeat as the Romans were greatly outnumbered. Caesar (whose self-reported casualty numbers are likely much lower than the actual amount) claims that 700 men died including 46 centurions. Caesar withdrew from his siege and Vercingetorix's victory attracted many new tribes to his cause. So too, however, did the Romans who convinced numerous Germanic tribes to join them.
Vercingetorix marched the army he had gathered thus far, mainly cavalry, to intercept Caesar. The two armies met in the Battle of the Vingeanne, where Caesar won the subsequent victory defeating Vercingetorix's cavalry.

Siege: With his cavalry routed Vercingetorix withdrew towards the Mandubii oppidum of Alesia, in what would become the siege of Alesia. After the poor performance at Gergovia, a direct assault on the Gauls by Caesar was no longer a viable solution. Thus, Caesar opted simply to besiege the settlement and starve out the defenders. Vercingetorix was fine with this, as he intended to use Alesia as a trap to conduct a pincer attack on the Romans, and sent a call for a relieving army at once. Vercingetorix likely did not expect the intensity of the Roman siege preparations. Although modern archeology suggests that Caesar's preparations were not as complete as he describes, it is apparent that Caesar laid some truly incredible siege works. Over the span of a month, some 25 miles of fortifications were built. They included a trench for soldiers, an anti-cavalry moat, towers at regular intervals, and booby traps in front of the trenches. The fortifications were dug in two lines, one to protect from the defenders, and one to protect from the relievers. Archeological evidence suggests the lines were not continuous as Caesar claims, and made much use of the local terrain, but it is apparent that they worked. Vercingetorix's relieving army arrived quickly, yet concerted coordinated attacks by both the defenders and relievers failed to oust the Romans.
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.