Short answers, pulled from the story.
Avaricum was an oppidum, a large fortified settlement, located in the territory of the Bituriges Cubi tribe in ancient Gaul, near the site of present-day Bourges. It was the largest and best-fortified town in Bituriges lands, set on fertile ground protected on most sides by rivers and marshland.
Caesar's Roman army besieged Avaricum in the winter of 52 BC as part of the broader Great Gallic Revolt. After 25 days of construction on a massive siege terrace, Roman soldiers exploited a storm that drove Gallic sentries from the walls, scaled the fortifications, and sacked the town. Only 800 of Avaricum's estimated 40,000 inhabitants escaped the ensuing massacre.
Vercingetorix had ordered a scorched-earth policy to starve Caesar's army, but the Bituriges pleaded for their capital to be spared, arguing its defenses made it impregnable. Vercingetorix reluctantly agreed, making Avaricum the only major town and supply center left standing in the region.
Caesar brought eight legions to Avaricum, most probably understrength, giving him an estimated 25,000-30,000 legionaries, plus thousands of auxiliaries, allies, mercenaries, slaves, and camp followers.
Caesar's engineers built a siege terrace 330 feet wide and 80 feet high between two flanking ramp walls, with two towers housing battering rams positioned at the ends. The construction took 25 days under constant harassment from the town's defenders.
After the massacre at Avaricum, Caesar rested and resupplied his army at the town until early June of 52 BC. He then marched on Gergovia to pursue Vercingetorix, a campaign that eventually led to the decisive Battle of Alesia.