Explanation: During the Siege of Alesia, Julius Caesar encircled the Gallic king Vercingetorix, and built a wall around the town of Alesia to hem him in and prevent him from escaping. However, Caesar was made aware of a massive Gallic army coming to break the siege. Rather than lifting the siege, Caesar opted to build ANOTHER wall - this one to protect his troops from the Gallic army in the countryside. This left the Romans nestled in a little strip between the two forces, relying on their supplies to wait out both the forces in Alesia, and the massive army outside that couldn’t sustain itself on the countryside