Two thousand years ago, on a hillside in eastern Gaul, a Roman army under Julius Caesar trapped a Gallic army inside a hill town called Alesia, and then did something stranger than a siege. Caesar built two walls, one facing inward at the men he had caught, to hold them in, the other facing outward at the quarter of a million Gauls marching to break the ring, to keep them out. Miles of timber and ditch, raised in a few weeks, and the object of all that labor was a number: the count of futures still open to the men inside, which the walls drove down to one. The one left them, when the digging was done, was capture.