Within three weeks of his victory, the foundation rites of New Rome were performed, and the much-enlarged city was officially inaugurated on May 11, 330. It was an act of vast historical portent. Constantinople was to become one of the great world capitals, a font of imperial and religious power, a city of vast wealth and beauty, and the chief city of the Western world. Until the rise of the Italian maritime states, it was the first city in commerce, as well as the chief city of what was until the mid-11th century the strongest and most prestigious power in Europe.

Constantine’s choice of capital had profound effects upon the ancient Greek and Roman worlds. It displaced the power centre of the Roman Empire, moving it eastward, and achieved the first lasting unification of Greece. Culturally, Constantinople fostered a fusion of custom, art, and architecture from the eastern regions of the empire with the Classical forms of Roman antiquity. The religion was Christian, the organization Roman, and the language and outlook Greek. The concept of the divine right of kings, rulers who were defenders of the faith—as opposed to the king as divine himself—was evolved there. The gold solidus of Constantine retained its value and served as a monetary standard for more than a thousand years. As the centuries passed—the Christian empire lasted 1,130 years—Constantinople, seat of empire, was to become as important as the empire itself; in the end, although the territories had virtually shrunk away, the capital endured.

Constantine’s new city walls tripled the size of Byzantium, which now contained imperial buildings, such as the completed Hippodrome begun by Septimius Severus, a huge palace, legislative halls, several imposing churches, and streets decorated with multitudes of statues taken from rival cities. In addition to other attractions of the capital, free bread and citizenship were bestowed on those settlers who would fill the empty reaches beyond the old walls. There was, furthermore, a welcome for Christians, a tolerance of other beliefs, and benevolence toward Jews.

Constantinople was also an ecclesiastical centre. In 381 it became the seat of a patriarch who was second in honour to the bishop of Rome; the patriarch of Constantinople is still “first among equals” (primus inter pares) among the primates of the Eastern Orthodox Church. Constantine inaugurated the first ecumenical councils; the first six were held in or near Constantinople. In the 8th and 9th centuries Constantinople was the centre of the battle between the Iconoclasts and the defenders of icons. The matter was settled by the seventh ecumenical council against the Iconoclasts, but not before much blood had been spilled and countless works of art destroyed. The Eastern and Western wings of the church drew further apart, and after centuries of doctrinal disagreement between Rome and Constantinople a schism occurred in the 11th century. The pope originally approved the sack of Constantinople in 1204, then decried it. Various attempts were made to heal the breach in the face of the Seljuq threat to the city, but the divisive forces of suspicion and doctrinal divergence were too strong.

By the end of the 4th century, Constantine’s walls had become too confining for the wealthy and populous metropolis. St. John Chrysostom, writing at the end of that century, said many nobles had 10 to 20 houses and owned 1 to 2,000 slaves. Doors were often made of ivory, floors were of mosaic or were covered in costly rugs, and beds and couches were overlaid with precious metals.

The population pressure from within, and the barbarian threat from without, prompted the building of walls farther inland at the hilt of the peninsula. These new walls of the early 5th century, built in the reign of Theodosius II, are those that stand today.

In the reign of Justinian I (527–565) medieval Constantinople attained its zenith. At the beginning of this reign the population is estimated to have been about 500,000. In 532 a large part of the city was burned and many of the population killed in the course of the repression of the Nika Insurrection, an uprising of the Hippodrome factions. The rebuilding of the ravaged city gave Justinian the opportunity to engage in a program of magnificent construction, of which many buildings still remain.

In 542 the city was struck by a plague that is said to have killed three out of every five inhabitants; the decline of Constantinople dates from this catastrophe. Not only the capital but the whole empire languished, and slow recovery was not visible until the 9th century. During this period the city was frequently besieged—by the Persians and Avars (626), the Arabs (674 to 678 and again from 717 to 718), the Bulgars (813 and 913), the Russians (860, 941, and 1043), and a wandering Turkic people, the Pechenegs (1090–91). All were unsuccessful.

In 1082 the Venetians were allotted quarters in the city itself (there was an earlier cantonment for foreign traders at Galata across the Golden Horn) with special trading privileges. They were later joined by Pisans, Amalfitans, Genoese, and others. These Italian groups soon obtained a stranglehold over the city’s foreign trade—a monopoly that was finally broken by a massacre of Italians. Not for some time were Italian traders permitted once more to settle in Galata.

In 1203 the armies of the Fourth Crusade, deflected from their objective in the Holy Land, appeared before Constantinople—ostensibly to restore the legitimate Byzantine emperor, Isaac II. Although the city fell, it remained under its own government for a year. On April 13, 1204, however, the Crusaders burst into the city to sack it. After a general massacre, the pillage went on for years. The Crusading knights installed one of themselves, Baldwin of Flanders, as emperor, and the Venetians—prime instigators of the Crusade—took control of the church. While the Latins divided the rest of the realm among themselves, the Byzantines entrenched themselves across the Bosporus at Nicaea (now İznik) and at Epirus (now northwestern Greece). The period of Latin rule (1204 to 1261) was the most disastrous in the history of Constantinople. Even the bronze statues were melted down for coin; everything of value was taken. Sacred relics were torn from the sanctuaries and dispatched to religious establishments in western Europe.

In 1261 Constantinople was retaken by Michael VIII (Palaeologus), Greek emperor of Nicaea. For the next two centuries the shrunken Byzantine Empire, threatened both from the West and by the rising power of the Ottoman Turks in Asia Minor, led a precarious existence. Some construction was carried out in the late 13th and early 14th centuries, but thereafter the city was in decay, full of ruins and tracts of deserted ground, contrasting with the prosperous condition of Galata across the Golden Horn, which had been granted to the Genoese by the Byzantine ruler Michael VIII. When the Turks crossed into Europe in the mid-14th century, the fate of Constantinople was sealed. The inevitable end was retarded by the defeat of the Turks at the hands of Timur (Tamerlane) in 1402; but in 1422 the Ottoman sultan of Turkey, Murad II, laid siege to Constantinople. This attempt failed, only to be repeated 30 years later. In 1452 another Ottoman sultan, Mehmed II, proceeded to blockade the Bosporus by the erection of a strong fortress at its narrowest point; this fortress, called Rumelihisarı, still forms one of the principal landmarks of the straits. The siege of the city began in April 1453. The Turks had not only overwhelming numerical superiority but also cannon that breached the ancient walls. The Golden Horn was protected by a chain, but the sultan succeeded in hauling his fleet by land from the Bosporus into the Golden Horn. The final assault was made on May 29, and, in spite of the desperate resistance of the inhabitants aided by the Genoese, the city fell. The last Byzantine emperor, Constantine XI (Palaeologus), was killed in battle. For three days the city was abandoned to pillage and massacre, after which order was restored by the sultan.