The city, which had been rebuilt by Severus and named Anatonina after Antoninus extended 300m further to the west than the old city. The city walls built by Severus on the side facing inland extended from what is now Sirkeci to the Turbe neighborhood of the Cemberlitas district, and, curving eastwards, extended downwards towards the Sea of Marmara. Severus had started the construction of the Hippodrome (At Meydani) in 203 AD, but the work was not completed. The inside of the city was adorned with impressive civic buildings and public baths, temples and the Necropolis (graveyard) extended as for as the area between Cemberlitas and Beyazid. It is probable that in this period main streets lined with columns were built on either side of it. The most important of these was the main street of the city, known as Mese, which followed almost exactly the same route as the present Divanyolu (Yeniceriler) Avenue.

In deep excavations carried out inside the city, the remains of the Roman graveyard was seen at a depth of 8m. A large number of tombs and grave steles have been found between Cemberlitas and Beyazid during the last fifty years or so. The life of this city, founded some time between the end of the 2nd and the beginning of the 3rd century AD, circa 200 AD, was not to be a very long one. When the emperor Diocletianus abdicated in 305 AD the Roman empire entered a new period of crisis and in the war that ensued between Maximinus and Licinius, Maximinus captured Istanbul in the winter of 312-313. However, Maximinus was defeated in Thrace and the city fell into the hands of his rival, Licinius was defeated in 323 AD; first of all he fled to Istanbul, then to Kadikoy on the Asian side. There, his army in disarray, he finally surrendered in Nikomedia (Izmit). Constantine had for some time been of the opinion that a new capital needed to be chosen for the Empire, and he considered Troy, which is near to the present-day Canakkale, to be the most appropriate choice. However, he had seen many of the virtues of Istanbul during the course of the war against Licinius, and Constantine I, who was the sole candidate for ruler of the Roman state, came to the conclusion that Byzantion (Istanbul) was, in military, economic and internal political terms, the most suitable place.

(a) Military reasons

Istanbul was a city that could be easily defended against external threats coming from the north or from the east against the state. It had been seen in the battle against Licinius that the city was easy to defend. Moreover, it was a borders of the empire, and from where their raids could be stopped. The Sasani to the east were a considerable danger to the empire and the emperor Valerianus, who had been taken prisoner by the Sasani king, had been humiliated to the extent of being used as a mounting block by the latter.

(b) Economic reasons

Due to the fact that Istanbul lay on both the land and maritime trade routes, it served as a bridge in trade relations. It was an important crossroads on these routes and a great deal of revenue was obtained from tolls and customs duties.

(c) Political reasons

Istanbul was a suitable place for the measures to be taken against disturbances within the state and was a completely new, clean place far from the old centres of corruption. Constantine's claim to be a believing Christian was not altogether convincing but he took a number a decisions that meant that Christians, who had previously been subjected to terrible persecutions and tortures, would now be regarded with tolerance and that the Christian religion would henceforth be the official religion of the state. In Rome, which was loyal to the old polytheist religion, such an initiative would have been well nigh impossible.

Constantine began to rebuild Istanbul in 325 AD. The foundations of the city walls, which were to extend further to the west on the inland side, were laid on 26 November, 328. At this time Christianity, which was just beginning to spread, had been allowed to recruit new followers to defend it from its rivals and had moreover been made the official religion of the state; this led to the creation of Christian legend about this action of Constantine, who had in fact remained a pagan. An example of this is the belief that an angel had appeared to him and shown him the places through which the new city walls would pass on the western side.