Constantinople (Greek: Κωνσταντινούπολις, translit. Kōnstantinoúpolis; Latin: Cōnstantīnopolis) was the capital city of the Roman Empire (330–395), of the Eastern Roman (Byzantine) Empire (395–1204 and 1261–1453), of the brief Crusader state known as the Latin Empire (1204–1261) and of the Ottoman Empire (1453–1923). In 1923 the capital of Turkey, the successor state of the Ottoman Empire, was moved to Ankara and the name Constantinople was officially changed to Istanbul. The city is located in what is now the European side and the core of modern Istanbul. The city is still referred to as Constantinople in Greek-speaking sources.
In 324 ancient Byzantium became the new capital of the Roman Empire by Emperor Constantine the Great, after whom it was renamed, and dedicated on 11 May 330. From the mid-5th century to the early 13th century, Constantinople was the largest and wealthiest city in Europe. The city became famous for its architectural masterpieces, such as Hagia Sophia, the cathedral of the Eastern Orthodox Church, which served as the seat of the Ecumenical Patriarchate, the sacred Imperial Palace where the Emperors lived, the Galata Tower, the Hippodrome, the Golden Gate of the Land Walls, and the opulent aristocratic palaces lining the arcaded avenues and squares. The University of Constantinople was founded in the fifth century and contained numerous artistic and literary treasures before it was sacked in 1204 and 1453, including its vast Imperial Library which contained the remnants of the Library of Alexandria and had over 100,000 volumes of ancient texts. It was instrumental in the advancement of Christianity during Roman and Byzantine times as the home of the Ecumenical Patriarch of Constantinople and as the guardian of Christendom's holiest relics such as the Crown of thorns and the True Cross.
Constantinople was famed for its massive and complex defences. The first wall of the city was erected by Constantine I, and surrounded the city on both land and sea fronts. Later, in the 5th century, the Praetorian prefect Anthemius under the child emperor Theodosius II undertook the construction of the Theodosian Walls, which consisted of a double wall lying about 2 kilometres (1.2 mi) to the west of the first wall and a moat with palisades in front. This formidable complex of defences was one of the most sophisticated of Antiquity. The city was built intentionally to rival Rome, and it was claimed that several elevations within its walls matched the 'seven hills' of Rome. Because it was located between the Golden Horn and the Sea of Marmara the land area that needed defensive walls was reduced, and this helped it to present an impregnable fortress enclosing magnificent palaces, domes, and towers, the result of the prosperity it achieved from being the gateway between two continents (Europe and Asia) and two seas (the Mediterranean and the Black Sea). Although besieged on numerous occasions by various armies, the defences of Constantinople proved impregnable for nearly nine hundred years.
In 1204, however, the armies of the Fourth Crusade took and devastated the city, and its inhabitants lived several decades under Latin misrule. In 1261 the Byzantine Emperor Michael VIII Palaiologos liberated the city, and after the restoration under the Palaiologos dynasty, enjoyed a partial recovery. With the advent of the Ottoman Empire in 1299, the Byzantine Empire began to lose territories and the city began to lose population. By the early 15th century, the Byzantine Empire was reduced to just Constantinople and its environs, along with Morea in Greece, making it an enclave inside the Ottoman Empire; after a 53-day siege the city eventually fell to the Ottomans, led by Sultan Mehmed II, on 29 May 1453, whereafter it replaced Edirne (Adrianople) as the new capital of the Ottoman Empire.