The Puerta de Alcalá is a Neo-classical monument in the Plaza de la Independencia in Madrid. It is regarded as the first modern post-Roman triumphal archbuilt in Europe, older than the similar monuments Arc de Triomphe in Paris and Brandenburg Gate in Berlin.
Puerta de Alcalá was a gate of the former city walls built by Philip IV. It stands near the city center and several meters away from the main entrance to the Parque del Buen Retiro. The square is bisected by Alcalá Street, although the street does not cross through the monument, and it is the origin of the Alfonso XII, Serrano and Olózaga streets. Its name originates from the old path from Madrid to the nearby town of Alcalá de Henares.
Madrid in the late 18th century still looked like a somewhat drab borough, surrounded by medieval walls. Around the year 1774, king Charles III commissioned Francesco Sabatini to construct a monumental gate in the city wall through which an expanded road to the city of Alcalá was to pass, replacing an older, smaller, gate that stood nearby. It was inaugurated in 1778.
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.