The Siege of Alesia is among the most celebrated battles of Roman times and stands as one of Gaius Julius Caesar's finest victories. The battle was the last major engagement between the Gaul's and Rome, precipitating the end of the Gallic Wars and ending Celtic dominance of Western Europe.
Caesar and his legions had been engaging in a war of conquest in Gaul since 58 BC. The campaign had been an extraordinarily bloody one, and despite defeating many of the major tribes and gaining a shaky control over the region, a revolt ending up destroying a quarter of Caesar's troops. Despite being mercilessly crushed, the revolt convinced many Gauls that throwing off the Roman yoke might be possible.
This led to the rise of Vercingetorix of the Averni tribe, who was crowned king of all the Gauls in 52 BC. A general revolt broke, and many Roman officials, soldiers, and merchants were killed across the country. Caesar was in winter quarters in Cisalpine Gaul north of Italy and did not learn of the chaos immediately.
When word reached him, Caesar sent legions to the north to put down other tribes, and marched with the rest and his auxiliary cavalry in pursuit of Vercingetorix. After delivering several setbacks to Caesar, Vercingetorix thought a general battle would be too risky, and withdrew to the vast hill fortress town of Alesia.
Where Alesia was located is still a matter of dispute, but the conventional wisdom places it in Monta-Arieux in the French region of Burgundy. Alesia featured formidable walls, and was located on a plateau ideal for defense. Between Vercingetorix's army and the town's citizens, it held around 80,000 Gauls.
Caesar arrived at Alesia with an army of possibly 60,000 men. He quickly saw that a frontal assault was out of the question, and he began a vast series of siegeworks, seldom equalled in antiquity, to enclose the town and starve it out. Caesar ordered an 18 kilometer wall called a circumvallation built to completely surround the town, and it was completed in roughly 3 weeks.
The Gauls launched a series of raids in order to prevent the completion of wall, but all attempts were beaten back. A small force Gallic cavalry force managed to break through and escape, and Caesar knew it was only a matter of time until enemy reinforcements arrived.
He ordered the construction of second 21km wall called a contravallation that was built to protect from the outside against the inevitable Gallic relief force. It too was strongly reinforced with towers, pit traps, and ditches. Sections of moat were even made by diverting water from the local rivers.
The cruel realities of siege and starvation began to take their toll on Vercingetorix's Gallic defenders, and to stretch provisions they decided to expel their women, children, and invalids from the fortress with a plea that Caesar let them through the lines. Caesar ruthlessly refused, and the Gallic civilians were left to miserably starve in the no-man's land between the fortress and the Roman works.
A large Gallic relief force eventually arrived in September, and on the 30th an assault was launched against the Roman outer wall while Vercingetorix sallied out of the fortress and attacked the inner. After several days of mostly failed attacks, on the third day the Gauls breached the outer wall and the Romans defending it were on the verge of being overrun. Caesar personally led a large force out of the Roman lines and hit the outer Gallic force from the rear. Caught between two Roman forces, the Gauls broke and were routed with great slaughter.
Vercingetorix, knowing no more help was coming, surrendered the next day. The surviving defenders of Alesia were then sold into slavery and the fortress town was completely obliterated by the Romans as a warning. This marked the end of any organized resistance in Gaul, and it was to become a key Roman province.
The Siege of Alesia was a masterpiece of military engineering, and the monumental efforts of his men combined with Caesar's tactical brilliance allowed the Romans to defeat a much larger enemy army. It led to 500 years of Roman dominance in Gaul, and the great fame it gave Caesar helped further set the stage for the coming Roman civil war and Caesar's ascension to dictator and the end of the Roman Republic.