Constantinople was the capital city of the Byzantine (330–1204 and 1261–1453), and also of the brief Latin (1204–1261), and the later Ottoman (1453–1923) empires. It was reinaugurated in 324 AD from ancient Byzantium as the new capital of the Roman Empire by Emperor Constantine the Great, after whom it was named, and dedicated on 11 May 330 AD.
From the mid-5th century to the early 13th century, Constantinople was the largest and wealthiest city in Europe.
By the the time the Ottomans took Constantinople the Empire consisted of the immediate environs around the city, a few islands in the Aegean, and the Peloponnesus in southern Greece. The city itself had been wracked by war and decimated by plague. It was described by contemporaries as being a series of individual walled villages and fields all surrounded by the Theodosian walls. It’s likely that the population had fallen to around 30,000 people, far below the quarter million people it had even just a century before, let alone the half a million to million people it had held at its height.
Constantinople was famed for its massive and complex defences. Although besieged on numerous occasions by various peoples, the defences of Constantinople proved invulnerable for nearly nine hundred years before the city was taken in 1204 by the Crusader armies of the Fourth Crusade, and after it was liberated in 1261 by the Byzantine Emperor Michael VIII Palaiologos, a second and final time in 1453 when it was conquered by the Ottoman Sultan Mehmed II. 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 km (1.2 miles) 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 that was engendered by its being the gateway between two continents (Europe and Asia) and two seas (the Mediterranean and the Black Sea).
The city was also famed for its architectural masterpieces, such as the Greek Orthodox cathedral of Hagia Sophia, 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.
Constantinople never truly recovered from the devastation of the Fourth Crusade and the decades of misrule by the Latins. Although the city partially recovered in the early years after the restoration under the Palaiologos dynasty, the advent of the Ottomans and the subsequent loss of the Imperial territories until it became an enclave inside the fledgling Ottoman Empire rendered the city severely depopulated when it fell to the Ottoman Turks, whereafter it replaced Edirne (Adrianople) as the new capital of the Ottoman Empire.