Constantinople (Κωνσταντινούπολις) or Byzantium (Βυζάντιον): Greek city on the Bosphorus, capital of the Byzantine Empire, modern İstanbul.
After 324, Constantine the Great expanded the old city of Byzantium to the west, naming the refounded town Constantinople. It was a success and by 328, the emperor decided to make it his capital. By then, the mighty walls already surrounded an area of 6 km². Soon, the city expanded beyond these fortifications: during the reign of Theodosius the Great (378-395), the suburb known as Kainopolis stretched forward along the Via Egnatia for almost 2½ km outside the walls of Constantine. To mark the true beginning of the urban area, Theodosius built the triumphal arch that was soon known as Golden Gate. The occasion may have been his victory over the Visigoths in 386, which did much to restore Roman self-confidence after the disastrous battle of Adrianople (378).
Of course an isolated triumphal arch does not defend an entire suburb, and after Rome had been captured and sacked by Alaric's Visigoths, the emperor Theodosius II ordered his praetorian prefect, Anthemius, to build new walls: these Theodosian Land Walls, one of the greatest pieces of military architecture ever, was built between 412 and 414, and were in 447 further expanded by another praetorian prefect of Theodosius's, Cyrus of Panopolis. The Arch of Theodosius was included in this line of fortifications.
The Golden Gate was the splendid entrance to the city for all visitors approaching the city from the west. Theodosius was not the last one to stage a triumphal entry of Constantinople over here; for example, on 14 September 628, the emperor Heraclius, who had decisively defeated the Sasanians and had recovered the True Cross, entered the city over here in a chariot drawn by four elephants. The arch was indeed the perfect place for celebrations: part of the walls was covered with gilded plates of bronze and there were all kinds of colorful statues.
The gate was later included in a fort with five towers by the emperors John I Tzimiskes (969-976) and Manuel I Comnenus (1143-1180). It was partly demolished when the Crusaders sacked Constantinople in 1204, but restored by John VI Palaeologus (1347-1454) and his regent and successor John V. The name Heptapyrgion, "the seven-towered bulwark", dates from this time. The fort was destroyed for the second time in 1391, when the Ottoman sultan Bayezid I ordered the emperor to do so, threatening with harsh measures against John V's captive son. After Mehmet II the Conqueror had become master of Constantinople in 1453, he rebuilt the Heptapyrgion; the Turkish name Yedikule is a translation of "seven towered bulwark". It was used as the Ottoman state treasury until 1789.
Today, the central square is in use as an open-air theater, and although the remains of the fort are impressive, the Golden Gate itself is a bit disappointing, as it has been closed for a long time.