The last great defensive work of the ancient world—and the first of the medieval age—was built to protect the city of Constantinople. The new Rome has been known by many names—Byzantion (the original Greek port), Constantinople (Constantine’s city), or later Byzantium, and today Istanbul. For his new capital city, Constantine selected a small Greek port, Byzantion, on a peninsula. The sea and a sea wall protected most of the city, but the land side of the peninsula required a heavier defensive system. When the emperor Theodosius expanded the city early in the fifth century, he ordered double walls and a moat for the vulnerable side. The Theodosian walls stood as a model for medieval builders. So effective were the walls that they protected the city until 1453 when the Turks blasted through them using cannon. Byzantine success depended on a system that combined vertical defense with defense in depth, that is, high walls and towers with double walls and a moat. The walls were built of stone and concrete and bonded with layers of brick, creating a colorful banded effect. The great inner wall was fifteen feet, six inches thick and had ninety-six towers, each of which could become an independent fortress. No one was allowed to build next to the wall; consequently, no traitor could bore through the walls from the back of his house. The open space also permitted rapid deployment of troops along the wall. Beyond the great wall, Theodosius’ engineers constructed a lower second wall about six feet, six inches thick; men on the high inner wall could see and shoot over the heads of those on the lower outer wall. A moat (water-filled ditch) defended the outer wall from anyone attempting to tunnel under it or batter it down. The moat was deep and wide and reinforced by low walls. To review: an attacking army faced a triple line of defense. If they were able to bridge the moat, they had to break through a wall, only to be trapped in front of an even stronger wall overlooked by fortress-towers. Such elaborate defenses required the vast resources of an empire to build. No western prince could afford the materials and the crew of skilled masons necessary to replicate the walls of Constantinople. Not until the twelfth century did such complex defensive structures appear in the West.