Battle of Alesia

01Caesar's inner wall (contravallation) was 11 miles; his outer wall (circumvallation) was 14 miles — both built in under six weeks.
02The relief army of roughly 250,000 Gauls attacked both walls at once; Caesar personally rode to plug a breach, his red cloak used as a field signal.
03When the relief army failed, Vercingetorix surrendered 80,000 starving survivors — and the 20,000 Mandubii civilians Caesar had refused to feed died in the gap between the walls.

A barrel-type butter churn

Caesar built two concentric walls totaling 25 miles — one facing inward to trap 80,000 men, one facing outward against 250,000 relieving warriors. He was the besieged and the besieger simultaneously.