The Roman aqueduct of Segovia was built, probably in the 1st century BC, to bring water from the mountains to the hilltop settlement of Segovia. It was a massive feat of engineering as it ran for around 15 km and had to cross a wide valley before it entered the city. It was used to bring water to the town until the 19th century.
Today the aquduct is part of a UNESCO World Heritage Site that also includes the old city and fortifications of Segovia. It is an enormous construction of masonry, 813 m in length, consisting of four straight segments and two superimposed arcades borne by 128 pillars. At the lowest point of the valley, the Aqueduct stands at a height of 28.5 m above ground. The tall arches of the aqueduct bridge are a major tourist attraction.
The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.