The siege of Avaricum (c.March-April 52 B.C.) was the first major clash between Julius Caesar and Vercingetorix during the Great Gallic Revolt, and ended with a Roman victory and the sack of the town. At the start of the great Gallic revolt Vercingetorix had hoped to prevent Caesar from joining his legions in their winter quarters, but when that failed the Gauls moved to besiege the town of Gorgobina. Caesar was forced to bring his legions out of their winter quarters to prevent the fall of this town, which had been settled in 58 B.C. by the Boii after the defeat of the Helvetii. Caesar captured Vellaunodunum, Cenabum and Noviodunum. Vercingetorix was forced to abandon the attack on Gorgobina, but he was unable to prevent the fall of Noviodunum. After the capture of that town Caesar moved on to besiege Avaricum.
Caesar was aware that Avaricum (modern Bourges) was the biggest and best fortified town of the Bituriges, and he believed that if he could capture the town then the entire tribe would surrender. Vercingetorix didn't share this opinion. At a council of war he persuaded the Bituriges to burn most of their towns and villages to prevent the Romans from finding any supplies. He also wanted them to destroy Avaricum, but the Bituriges convinced the council that they could defend their town, and against his better judgement Vercingetorix agreed to garrison the town.
Avaricum was easy to defend. The town was protected by a river and a sizable marsh, and there was only one narrow approach to the town. Caesar camped outside that narrow entrance, and began to build a giant mound and siege towers. The marshy location of the town prevented the Romans from building their normal line of circumvallation by the river and marsh.
Vercingetorix followed behind the Romans, and set up his own camp fifteen miles from the town. This camp was also protected by swamps, which prevented the Romans from attacking it. Scouts keep the two Gallic forces in touch with each other while Vercingetorix concentrated on attacking any Roman foraging party that travelled too far from their main camp. The Romans soon ran short of supplies, partly because of these attacks, but more worryingly because the Aedui, Rome's most valuable allies in Gaul, were reluctant to provide supplies.
When he discovered that the Romans had completed their siege towers, Vercingetorix moved his camp nearer to the town and prepared to ambush the next days' foraging party. When Caesar's scouts reported this move he decided to attack the new camp. Early on the following day, while Vercingetorix was waiting in vain for the Roman forages, Caesar and the main Roman army advanced towards his camp, where they discovered the Gallic army formed up on a hill. For a short time it looked as if a major battle was about to break out, but the two armies were separated by a swamp and neither side was willing risk moving first. Eventually the Romans returned to their camp. When Vercingetorix returned to his camp he was accused of planning to betraying his army, and was forced to defend his actions.
The siege lasted for twenty seven days. The Gauls had become much more skilled at defending their towns against Roman siege engines, and many of the inhabitants of Avaricum were experienced iron miners, which gave them the skills needed to counter the Roman mound. When the Romans tried to use grappling hooks to pull stones off the walls the Gauls trapped them and used their own machines to drag the grappling hooks inside the city. When the Romans attempted to dig tunnels under the walls the Gallic miners dug their own countermines.
After twenty five days the Roman mound was 330 feet wide and 80 high, and was getting close to the city walls. Just after midnight the Romans realised that the mound was sinking. The Gauls had dug tunnels under the mound and had set fire to the pit props, collapsing the tunnel. The mound must have had some wooden supports, for the Gauls then poured pitch on it from above in an attempt to burn down the mound, while at the same time they launched sallies from gates on either side of the mound. The lateness of the hour and the flames caused great confusion in the Roman camp, but eventually, with the help of the entire besieging army, the situation was restored and the Gallic attack failed.
On the following day the garrison of Avaricum decided to attempt to escape from the town and cross the marsh to join Vercingetorix. This plan required secrecy, but the Romans were alerted by the sounds of arguments coming from within the town, where the women of Avaricum were pleading with soldiers not to abandon them to the Romans. Aware that secrecy was lost the garrison abandoned the evacuation plan, but the disturbance helped convince Caesar that it was time to attack the town.
On the next day, under the cover of a storm, the Romans successfully reached the top of the town walls. The Gauls formed up in a wedge in the open marketplace, ready to resist the expected Roman attack, but instead of climbing down into the town the Roman infantry spread out along the top of the walls. This unnerved the defenders, who believed that they were about to be trapped in the city. The wedge broke up and the Gallic soldiers attempted to break out of the city gates. Some were killed by the Roman infantry in the narrow approaches to the gate and most of the rest were caught by the cavalry outside the town.
The fall of the Avaricum was followed by a massacre of the inhabitants, woman and children included. Caesar described this as having been caused by a combination of anger at the massacre of the Romans at Cenabum and frustration after the long difficult siege, but his brief description gives no indication of his attitude to this massacre (normal practise was to enslave the population of a captured city, with most of the money going to the commander of the victorious army).
The fall of Avaricum didn't have the effect Caesar had hoped. Vercingetorix managed to restore the morale of his army with a rousing speech, and he was soon able to replace the troops lost during the siege. More importantly the Aedui finally abandoned their long attachment to the Roman cause and joined the revolt. Caesar lost one of his best sources of cavalry, and faced an ever more powerful coalition of Gallic tribes. His next move, an attack on Gergovia, ended with his only major defeat at the hands of the Gauls, but Vercingetorix then attempted to defend Alesia, a move that gave Caesar a chance to defeat the Gallic army in a single location.