Explanation: During the Siege of Alesia, Gaius Julius Caesar, of later dictator fame, besieged the Gallic leader Vercingetorix by building a wall around the walled city of Alesia.

When Caesar heard that a massive Gallic army was coming to reinforce Vercingetorix, instead of lifting the siege, Caesar built ANOTHER wall - this one around his siege camp, to keep the reinforcing Gauls out.