The Puente Romano (Spanish for Roman Bridge) is a Roman bridge over the Guadiana River at Mérida.
It is the world's longest (in terms of distance) surviving bridge from ancient times, having once featured an estimated overall length of 755 m with 62 spans. Today, there are 60 spans (three of which are buried on the southern bank) on a length of 721 m between the abutments. Including the approaches, the structure totals 790 m. It is still in use, but was pedestrianized in 1991 as road traffic was redirected to use the nearby Lusitania Bridge.
Annexed to the bridge is the Alcazaba of Mérida, a Moorish fortification built in 835. Close to the remains of the Acueducto de los Milagros, there exists another Roman bridge at Mérida, the much smaller Puente de Albarregas.
References: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.