The siege of Noviodunum (probably March 52 B.C.) was the third of three Roman attacks on Gallic towns that forced Vercingetorix to abandon his siege of Gorgobina. It also saw the first direct clash between the main armies of Caesar and Vercingetorix, a minor cavalry action fought outside the town.
At the start of the great Gallic revolt Vercingetorix had hoped to prevent Caesar from travelling from Italy to his legion's winter quarters in the north of Gaul. When this effort failed Vercingetorix moved to besiege Gorgobina. This town, somewhere in the lands of the Aedui, had been settled by the Boii, one of the tribes that had been involved in the migration of the Helvetii in 58 B.C. At the end of that campaign the Aedui had asked Caesar to allow the Boii to settle in their lands in an attempt to restore some of the strength they had lost at the hands of the Sequani and their German allies.
This meant that Gorgobina and its inhabitants were Caesar's allies. Caesar was faced with a difficult decision. The campaign was taking place late in the winter of 53-52 B.C., much earlier than the Romans would normally have left their winter camps, and supplies were still short, but if the Romans remained in their camps and let Gorgobina fall then Caesar would have lost the confidence of his remaining allies, and a dangerous revolt would have spread even faster.
Caesar decided to attempt to lift the siege. Leaving two legions at Sens (Agendicum) he moved south, capturing the towns of Vellaunodunum and Cenabum (Orleans), before moving on to besiege Noviodunum (normally identified with Neung sur Beuvron, just over twenty miles to the south of Orleans).
Caesar doesn't say how long the siege of Noviodunum lasted, but it probably wasn’t very long before ambassadors from the town asked for surrender terms, as Caesar agreed to pardon the inhabitants for their part in the revolt so that he could 'execute the rest of his designs with the rapidity by which he had accomplished most of them'. The inhabitants were ordered to surrender all of their arms and their horses and to provide a number of hostages.
Caesar's rapid advance from Agendicum forced Vercingetorix to abandon the siege of Gorgobina and move towards Noviodunum in an attempt to lift the siege, but by the time his cavalry came in sight of the town the citizens had already agreed to surrender. The first batch of hostages had been delivered and a number of centurions and legionaries were inside the town.
When the inhabitants of Noviodunum spotted Vercingetorix's cavalry in the distance a mob seized the walls, closed the gates and prepared to defend the city, in the hope that the main Gallic army was close behind. Realising what was happening, the centurions within the city managed to recapture the gates and the Roman troops within the city were evacuated safely.
Caesar responded to the new threat by sending most of his horsemen to attack the Gauls, triggering a cavalry battle, which the Gauls soon began to win. This forced Caesar to send in his cavalry reserve, a force of 400 German auxiliaries that he had recruited when the revolt reduced his supply of Gallic cavalry. The Germans turned the tide of the battle and Vercingetorix's men were forced to retreat all the way back to the main army.
The defeat of the relief effort must have come as a great shock to the inhabitants of Noviodunum, who now found themselves facing a potentially very angry Caesar having broken the terms of their surrender. In an attempt to rescue the situation the townsmen arrested a number of people who were accused of having led the mob, and turned them over to Caesar. Caesar doesn't actually record the fate of the town, instead stating that 'when these affairs were accomplished, Caesar marched to the Avaricum'. This might suggest that the original surrender terms were respected, particularly as the mob that forced the Romans out of the town doesn't seem to have killed any Roman soldiers. Caesar's main interest at this point was the capture of Avaricum, the largest town of the Bituriges.