Capital of the Roman Empire (324 - 395) 

The lands of the Roman Empire stretched from the Atlantic in the west to the Euphrates and the Tigris to the east, and early in the fourth century the idea of establishing a second capital to control the eastern provinces had germinated. Byzantium, strategically positioned at the crossroads of the land and sea trade routes between east and west, was the obvious choice. This new status underscored the city's significant cultural and political position in the Old World.

Constantine I the Great (324-337) invited high-born Romans to settle in Byzantium, so swelling the Roman population. At the same time he launched a building programme to befit the city for its new role as eastern capital. The harbours and water supply channels were improved, and construction commenced of a new water distribution system within the city. A new wall was built to improve the city's defences.

The Hippodrome begun by Septimus Sevenıs was completed. This great building, 117 m wide and 480 m long, could seat 100,000 people. Down the centre was the spina, around which the chariots raced. As well as chariot racing, the Hippodrome was used for wild animal fights, athletic competitions, festivals, celebrations and entertainments. It was mainly here that the ordinary people got the chance to see and be with the emperor. The most exciting events of all were the chariot races between four teams, the Blues representing the air, the Whites water, the Greens earth, and the Reds fire. On the walls of the Hippodrome stood numerous statues, most famous of which were the four bronze horses later carried back to Venice by the Latin invaders and installed in St. Mark's Square.

The imperial palace was next to the Hippodrome on the site where Sultanahmet Mosque now stands, and the area where Topkapı Palace was later built was the ancient acropolis with its monumental temples.

Known earlier as Nea Roma, Constantine I named it Constantinople after himself on 11 May 330.

The same year he built the Forum Constantine (now Çemberlitaş Square), and had a bronze statue of himself placed on top of the tall column brought here from the Temple of Apollo in Rome. The 35 m high column was badly damaged at an early date, and iron hoops placed around it in the early 5th century. As a result the Turks referred to it as the Hooped Stone or Çemberlitaş.

Constantine I erected the Milion Stone which was the symbolic hub of all roads fanning out through the Eastern Roman Empire, into Russia, Persia, Egypt and Europe. Just as all roads had earlier led to Rome, they now led to Constantinople, and merchants from a myriad countries found their way here from the remotest corners of the world.

When Christianity developed into a religion based on the figure of Christ and his divine mission, the concept of the church arose. Haghia Eirene, the church of the Divine Peace, was one of the oldest Eastern Roman churches, and took its present form when it was enlarged during the reign of Constantine I. Before Haghia Sophia was constructed this was the patriarchal cathedral. After the Turkish conquest it was used as an armoury by the janissaries, and housed Turkey's first military museum established in the nineteenth century. It stands in the first courtyard of Topkapı Palace.

Haghia Sophia, the largest and most magnificent of the eastern churches, was first built in 360 by Constantine I. Although the patriarch of Constantinople was the nominal head of the Orthodox Church, all authority lay with the emperor.

The city's infrastn.ıcture quickly became inadequate for the city as its population grew, and in 375 the Emperor Valens (364-378) constructed the 1000 m long Valens Aqueduct as part of a new water supply system over the valley west of the Hippodrome. Water from the Belgrade Forest beyond the city was carried over the aqueduct to the centre of the city around the Great Palace.

Several sets of walls were built around the city, beginning with the time of its founder Byzas, and they enclosed areas of differing size. Beyond the outer wall was a moat 10 m deep and 20 m wide, and inside this a second wall with 96 towers. As well as gates used by the general public, there were others reserved for military purposes. The walls overlooking the mouth of the Golden Horn where the city was least wlnerable to attack were the weakest. The next section to the south were the walls along the Marmara Sea which were 8260 m long and pierced by the Ahırkapı, Çatladıkkapı, Samatya and Narlıkapı gates. The land walls were 5632 m long and contained the Belgrad, Silivrikapı, Mevlevihane, Topkapı, Edirnekapı, Eğrikapı and Yedikule gates.

Yedikule Gate was also known as Porta Aurea or the Golden Gate, and was the most magnificent, consisting of three archways. It was built by Emperor Theodosius (379-395). Over the gateway was a double headed Byzantine eagle carved in relief. It was through this gate that the emperors passed when returning from victorious campaigns. Istanbul's city walls were almost invincible, and only breached twice in their entire history, once in 1204 by the Fourth Crusaders and once in 1453 by the Turks.

In 390 the Emperor Theodosius I had an obelisk brought from Egypt to Istanbul which he intended to erect as a mark of Roman supremacy. The obelisk dated from 1500 BCduring the reign of Pharaoh Tuthmosis II, and was one of two which stood at the entrance of the Luxor Temple in the city of Teb. The hieroglyphic inscriptions on the obelisk tell of sacrifices made to the god Amon-Ra. The obelisk was placed on the spina in the Hippodrome, on a rectangular marble plinth bearing relief carvings depicting Theodosius watching chariot races in the Hippodrome, and scenes showing how the obelisk was set in place.

Another monument on the spina of the Hippodrome was a bronze statue of three entwined serpents brought from the Temple of Apollo in Delphi. It had been made from the shields of Persian soldiers killed in the Battle of Palatea. Originally there was a gold cauldron resting on the heads of the three serpents, but this was apparently melted down for minting coins during the Latin occupation of the city, along with the bronze plates which covered the third of the ancient monuments on the spina, a stone pillar 32 m in height.