Julius Caesar won the Siege of Alesia with a military tactic known as investment: build walls around the besieged settlement’s own walls, and and then build another layer of walls around those ones.
Alesia was the decisive victory that gave Julius Caesar control over Gaul (what is today France, Belgium, Luxembourg, and bits of the surrounding countries). In the so-called Gallic Wars, Caesar aimed to draw these territories into the nascent Roman Empire. The Gauls, united under a charismatic chieftain named Vercingetorix, led a revolt against Caesar’s troops.
Vercingetorix had a large army: at the time about 30,000 strong. In 52 BCE Caesar made an assault against Vercingetorix’s home town Gergovia, and failed miserably. The chieftain was good on the defence, and the Roman legions weak on the attack. So, when Vercingetorix’s army moved to Alesia, Caesar did not try to assault them directly. Instead, he surrounded the fortified town and began a siege.
However, a siege is just what Vercingetorix wanted. By this time his army had swollen to perhaps 80,000 warriors, and tribes from around Gaul were gathering to come to his defence. When they arrived, he could sortie out of the besieged town and crush the Romans between his two armies.
Caesar’s response has gone down in military history. First, he built a wall between his armies and the town. The technical term for this is circumvallation. This wall was enormous – 16km long – and went up startlingly quickly. Then, to defend against the anticipated attack from without, he built another wall on the other side of his troops – a contravallation. This one was more than 20km long.
Both the circumvallation and the contravallation were heavily fortified. The layout was simple: the walls of Alesia, surrounded by Caesar’s first wall, surrounded by the Roman armies, surrounded by another wall.
When the relief force arrived, it was huge. Caesar estimated a quarter of a million troops – although he was almost certainly exaggerating that number. But all of them were effectively walled out of the siege. Vercingetorix tried his pincer manoeuvre, but both the Gauls within and the Gauls without were unable to penetrate Caesar’s fortifications.
The Gallic army disintegrated. Vercingetorix surrendered – famously laying his arms down at Caesar’s feet. Vercingetorix remained a prisoner of Rome for the rest of his life; he was executed five years later. Gaul became part of the Roman Empire, and Caesar became its first emperor.