Constantine the Great effectively became the emperor of the whole of the Roman Empire in September 324.
Two months later, he laid out the plans for a new, Christian city to replace Byzantium.
As the eastern capital of the empire, the city was named Nova Roma; most called it Constantinople, a name that persisted into the 20th century.
On 11 May 330, Constantinople was proclaimed the capital of the Roman Empire, which was later permanently divided between the two sons of Theodosius I upon his death on 17 January 395, when the city became the capital of the Eastern Roman (Byzantine) Empire.

The establishment of Constantinople was one of Constantine's most lasting accomplishments, shifting Roman power eastward as the city became a center of Greek culture and Christianity.
Numerous churches were built across the city, including Hagia Sophia which was built during the reign of Justinian the Great and remained the world's largest cathedral for a thousand years.
Constantine also undertook a major renovation and expansion of the Hippodrome of Constantinople; accommodating tens of thousands of spectators, the hippodrome became central to civic life and, in the 5th and 6th centuries, the center of episodes of unrest, including the Nika riots.
Constantinople's location also ensured its existence would stand the test of time; for many centuries, its walls and seafront protected Europe against invaders from the east and the advance of Islam.
During most of the Middle Ages, the latter part of the Byzantine era, Constantinople was the largest and wealthiest city on the European continent and at times the largest in the world.

Constantinople began to decline continuously after the end of the reign of Basil II in 1025.
The Fourth Crusade was diverted from its purpose in 1204, and the city was sacked and pillaged by the crusaders.
They established the Latin Empire in place of the Orthodox Byzantine Empire.
Hagia Sophia was converted to a Catholic church in 1204.
The Byzantine Empire was restored, albeit weakened, in 1261.
Constantinople's churches, defenses, and basic services were in disrepair, and its population had dwindled to a hundred thousand from half a million during the 8th century.
After the reconquest of 1261, however, some of the city's monuments were restored, and some, like the two Deisis mosaics in Hagia Sofia and Kariye, were created.

Various economic and military policies instituted by Andronikos II, such as the reduction of military forces, weakened the empire and left it vulnerable to attack.
In the mid-14th-century, the Ottoman Turks began a strategy of gradually taking smaller towns and cities, cutting off Constantinople's supply routes and strangling it slowly.
On 29 May 1453, after an eight-week siege (during which the last Roman emperor, Constantine XI, was killed), Sultan Mehmed II "the Conqueror" captured Constantinople and declared it the new capital of the Ottoman Empire.
Hours later, the sultan rode to the Hagia Sophia and summoned an imam to proclaim the Islamic creed, converting the grand cathedral into an imperial mosque due to the city's refusal to surrender peacefully.
Mehmed declared himself as the new "Kaysar-i Rûm" (the Ottoman Turkish equivalent of Caesar of Rome) and the Ottoman state was reorganized into an empire.
Constantine the Great effectively became the emperor of the whole of the Roman Empire in September 324. Two months later, he laid out the plans for a new, Christian city to replace Byzantium. As the eastern capital of the empire, the city was named Nova Roma; most called it Constantinople, a name that persisted into the 20th century. On 11 May 330, Constantinople was proclaimed the capital of the Roman Empire, which was later permanently divided between the two sons of Theodosius I upon his death on 17 January 395, when the city became the capital of the Eastern Roman (Byzantine) Empire.
The establishment of Constantinople was one of Constantine's most lasting accomplishments, shifting Roman power eastward as the city became a center of Greek culture and Christianity. Numerous churches were built across the city, including Hagia Sophia which was built during the reign of Justinian the Great and remained the world's largest cathedral for a thousand years. Constantine also undertook a major renovation and expansion of the Hippodrome of Constantinople; accommodating tens of thousands of spectators, the hippodrome became central to civic life and, in the 5th and 6th centuries, the center of episodes of unrest, including the Nika riots. Constantinople's location also ensured its existence would stand the test of time; for many centuries, its walls and seafront protected Europe against invaders from the east and the advance of Islam. During most of the Middle Ages, the latter part of the Byzantine era, Constantinople was the largest and wealthiest city on the European continent and at times the largest in the world.
Constantinople began to decline continuously after the end of the reign of Basil II in 1025. The Fourth Crusade was diverted from its purpose in 1204, and the city was sacked and pillaged by the crusaders. They established the Latin Empire in place of the Orthodox Byzantine Empire. Hagia Sophia was converted to a Catholic church in 1204. The Byzantine Empire was restored, albeit weakened, in 1261. Constantinople's churches, defenses, and basic services were in disrepair, and its population had dwindled to a hundred thousand from half a million during the 8th century. After the reconquest of 1261, however, some of the city's monuments were restored, and some, like the two Deisis mosaics in Hagia Sofia and Kariye, were created.
Various economic and military policies instituted by Andronikos II, such as the reduction of military forces, weakened the empire and left it vulnerable to attack. In the mid-14th-century, the Ottoman Turks began a strategy of gradually taking smaller towns and cities, cutting off Constantinople's supply routes and strangling it slowly. On 29 May 1453, after an eight-week siege (during which the last Roman emperor, Constantine XI, was killed), Sultan Mehmed II "the Conqueror" captured Constantinople and declared it the new capital of the Ottoman Empire. Hours later, the sultan rode to the Hagia Sophia and summoned an imam to proclaim the Islamic creed, converting the grand cathedral into an imperial mosque due to the city's refusal to surrender peacefully. Mehmed declared himself as the new "Kaysar-i Rûm" (the Ottoman Turkish equivalent of Caesar of Rome) and the Ottoman state was reorganized into an empire.
Istanbul