Explanation: During Julius Caesar's Gallic Wars, taking place in what-is-now-France, Caesar and the Roman forces besieged the Gallic coalition leader Vercingetorix in Alesia. To keep him from slipping past their lines, they built a (wooden) wall around the city they were besieging.

... then they heard a massive Gallic army was marching to lift the Roman siege.

So Caesar ordered all the crops in the region to be harvested at once, what could not be carried to be burned, and a second wall to be built - this one around his siege camps to keep the enemy Gauls out!

Nice and snug!

Since his forces had all the local supplies, and the Gallic relief force was huge, both the reinforcing army and Vercingetorix's garrison ran out of supplies before they could force Caesar from his defensive positions, giving Caesar his crowning (POETIC LICENSE) achievement of the Gallic Wars, and effectively ending the wars in the favor of the Roman Republic.