When the major projects had been completed in the completely rebuilt and enlarged city, an impressive opening ceremony was held on 11 May, 330 AD. The city had been granted all the privileges that belonged to Rome and its governor bore the title of proconsul. From 359 onwards this post was replaced by that of a person who was both governor and head of the municipality responsible for the administration of the city, and he was referred to as prafectus' (Epakhos in Greek), or prefect. The names of these prefects of old can still be read in the inscriptions on certain works belonging to the Roman period. (For example on the Dikilita~ pedestal, on the Mevlevihane gate of the city and on an obelisque known as Kizta~i, located in what was formerly the slave market of Istanbul). The city was divided into fourteen zones, twelve of which were within the city walls, the thirteenth at Galata and the fourteenth in the Blakherna district on the ECrikapi side. There is a list of all the buildings in each of these zones and the divisions of each zone in an old document. Due to the fact that no trace is left of Severuss walls, or of the new walls extending a further 250Cm to the west, the route followed by these walls is not known. Although it is alleged that the name of the Isakapisi or Esekaptst district to the west of Cerrahpa~a was taken from a piece of Constantine's wall, which was intact until 1509, and from one of its gates, there is no scientific basis for this theory. It is assumed that these walls followed the Golden Horn from Ayakapisi or Elnkapani as far as Fatih (according to some as far as Sultan Selim) and from this point continue downwards towards the Bayrampasa Creek then, passing through the lsakapisi district, follow a route to the east of Samatya and extend as far as the Sea of Marmara. The Emperor Constantine had a large forum, which was round or oval in shape, built in the centre of the city; in the centre of this forum was his own statue, placed on top of a column of reddish stone. This column is today known as Cemberlitas. The statue on the top of the column represented Constantine as Apollo saluting the sun. When the city was rebuilt, the Great Palace was constructed on the slopes of Sultanahmet overlooking the Sea of Marmara; this building was constantly added to by various emperors until the 11th century amd became a veritable "city within a city".

The Senate and the Hippodrome were completed and monuments were brought from overseas to adorn the latter. A number of temples, and, according to Christian belief, some large churches were built as well. Leading Roman statesmen were encouraged to settle in the city and contribute to its development. One of these was Philozenus, who had the cistern known as Binbir Direk (one thousand columns) and the palace above it built; another was Antioch, who commissioned a private residence, the ruins of which can be seen today between the street known as Divanyolu and the present Central Law Courts building. The emperors who succeeded Constantine continued to adorn the city with new buildings and structures. The most important of these is the water supply system built by Valens (364-378). The large aqueduct belonging to this system is still standing and is also known as the Bozdo5an Aqueduct. In 395 AD the city's biggest square, known as Theodosius or Tauros square, was built in what is now Bayezid. There was a gate leading into this square, which was 200m wide, and a number of monuments around it. One of these was a gigantic monumental entrance supported by four marble columns, the remains of which were discovered in 1956. Apart from this there was also a monument erected in the name of Theodosius I, the top of which could be reached by means of an inside staircase and the sides of which were adorned with reliefs depicting the battles fought by the emperor and his successes; this monument survived until the great earthquake of 1509. A few pieces of the reliefs adorning this monument can today be seen in the foundations of the Turkish bath at Bayezit.

Not long after, in 403 AD, another big forum and an impressive monument 70 metres in height were built on Istanbul's seventh hill, now the Cerrahpa~a district of the city, in the name of the emperor Arcadius. The console of this monument can still be seen. Although the statues of Arcadius on its top toppled over and were destroyed in a short space of time, the monument itself, the exterior of which was covered with marble reliefs, survived well into the Turkish period, until 1715 to be exact. However, in spite of all this development it was considered necessary to expand the city still further and at the beginning of the 5th century. in the reign of Theodosius H, the city walls of today were built, extending the boundaries of the city still further. In a treatise entitled Notitia urbis Constantinopolitinai', considered to have been written in the reign of Theodosius II, the fourteen divisions of the city and the important buildings in each of these divisions were stated; the names of private palaces and the number of buildings are given as well. This makes it possible to arrive at a rough estimate of the city's population at that time. The remains of one of these private residences (together with its mosaic floor) belonging to Princess Juliana Anicia, who is known to have lived at the beginning of the 6th century were discovered during excavations for the foundations of the municipal building carried out during the 1950's; unfortunately no effort was made to preserve them.