Constantinople, the modern Istanbul, was the capital of the Roman and Byzantine Empire from 330 until its takeover by the Ottoman Empire in 1453. After this, it continued to function as the Ottoman capital Konstantiniyye until the abolition of the Ottoman sultanate in 1922. It was located at the core of the modern city of Istanbul, on a peninsula at the southeastern tip of Thrace in Europe; with the Bosporus strait and the ancient cities of Chalcedon and Chrysopolis in Bithynia, Anatolia (Asia Minor) to the east; the Golden Horn and the citadel of Galata (Pera) to the north; the Sea of Marmara to the south; and the Princes' Islands to the southeast.
Constantinople was founded on the site of the existing city of Byzantium in 324, during the reign of Constantine the Great, initially as New Rome. In 330 it became the new capital of the Roman Empire, and continued in that role after the empire was divided in an Eastern and Western half and after the Western Roman Empire ended in the 5th century. The ascendancy of Constantinople marks the empire's "Byzantine" era.
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 Imperial Palace of the emperors; the Hippodrome; the Golden Gate of the Land Walls; and opulent aristocratic palaces. The University of Constantinople was founded in the 5th century and included a vast Imperial Library, which contained more than 100,000 volumes.
Constantinople was famous for its massive and complex fortifications, which ranked among the most sophisticated defensive architecture of antiquity; the Theodosian Walls 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. Constantinople's location between the Golden Horn and the Sea of Marmara reduced the land area that required defensive walls. The city was constructed intentionally to rival Rome, and it was claimed that several elevations within its walls matched Rome's seven hills. The impenetrable defenses enclosed magnificent palaces, domes, and towers, the result of the prosperity Constantinople achieved as 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 defenses of Constantinople proved impenetrable for nearly nine hundred years.
In 1204, during the Fourth Crusade, the city was sacked by the Latin Crusaders and became the capital of the short-lived Latin Empire (1204–1261). The Byzantine Empire of Nicaea recaptured the city in 1261. Under the following Palaiologan dynasty it enjoyed a partial recovery. With the advent of the Ottoman Empire in 1299, the empire's territories dwindled, and the city began to lose population. By the early 15th century, the empire was reduced to just Constantinople and its environs, along with the territories of the despotate of Morea, in Peloponnese, Greece, making it an enclave inside the Ottoman Empire. The city was finally besieged and conquered by the Ottoman Empire in 1453. It served as the Ottoman capital until the empire's dissolution and the establishment of the Republic of Turkey in 1923, when the capital was moved to Ankara. Now known internationally under its modern Turkish name Istanbul, it has become the most populous city in Europe, with a population of more than 16 million residents, and the economic and financial centre of Turkey, straddling the Bosporus strait and lying in both Europe and Asia.
Before Constantine I, the city was known as Byzantium (Greek Byzantion), the name associated with its original foundation as a Greek colony in the 7th century BC. It was briefly renamed Augusta Antonina in the early 3rd century AD by the Emperor Septimius Severus (193–211), who had razed it to the ground during the civil war of 196 AD, and then had it rebuilt in honour of his son Marcus Aurelius Antoninus, the future Emperor Caracalla. The name appears to have been quickly forgotten and abandoned, and the city reverted to Byzantium after either the assassination of Caracalla in 217 or, at the latest, the fall of the Severan dynasty in 235.
When Constantine transferred the capital of the empire to his newly refounded city in 330, he initially designated it "New Rome" (Nova Roma, Νέα Ῥώμη). During this time, it was also called 'Second Rome', 'Eastern Rome', and Roma Constantinopolitana ('Constantinopolitan Rome'). However, it soon became more widely known as Constantinople (Greek: Κωνσταντινούπολις, romanized: Kōnstantinoupolis, lit. 'city of Constantine') in honour of its founder.
As the city became the sole remaining capital of the Roman Empire after the fall of the West, and its wealth, population, and influence grew, the city also came to have a multitude of by-names, such as Basileuousa (Queen of Cities) and Megalopolis (the Great City). In colloquial speech, it was commonly referred to as just i Polis (ἡ Πόλις) 'the City' by Constantinopolitans and provincial Byzantines alike.
This designation was also the source of what was to become the city's modern Turkish name, Istanbul. It is derived from the Greek phrase eis tin Polin (εἰς τὴν Πόλιν, or in its spoken form in Medieval Greek: stin Poli(n) στην Πόλι(ν), both meaning 'in the City'. Variants of this name are first attested in languages such as Armenian and Arabic from the 10th century, before the name was adopted also by the Ottoman Turks.
During the Ottoman era, the city was referred to either as Istanbul or, in more elevated registers, as Kostantiniyye (قسطنطينيه), originally an Arabic calque of "Constantinople" meaning "city of Constantine". At some times during the Ottoman era, an adaptation of "Istanbul" as "Islambol" (meaning "full of Islam") was also common. Western languages mostly continued to refer to the city as Constantinople until the early 20th century. After 1930, the government of modern Turkey began to urge other countries to use Turkish names for Turkish cities, and the city came to be known as Istanbul and its variations in most world languages.
Constantinople was founded by the Roman emperor Constantine I (272–337) in 324 on the site of an already-existing city, Byzantium (Ancient Greek: Βυζάντιον, romanized: Byzántion), which was settled in the early days of Greek colonial expansion, around 657 BC, by colonists of the city-state of Megara, across from the town of Chalcedon on the Asiatic side of the Bosphorus.
Hesychius of Miletus wrote that some "claim that people from Megara, who derived their descent from Nisos, sailed to this place under their leader Byzas, and invented the fable that his name was attached to the city." Some versions of the founding myth say Byzas was the son of a local nymph, while others say he was conceived by one of Zeus' daughters and Poseidon. Hesychius also gives alternate versions of the city's founding legend, which he attributed to old poets and writers:
The city maintained independence as a city-state until it was annexed by Darius I in 512 BC into the Persian Empire, as he considered the site the optimal location to construct a pontoon bridge crossing into Europe, given that Byzantium was situated at the narrowest point in the Bosphorus strait. Persian rule lasted until 478 BC, when, as part of the Greek counterattack to the Second Persian invasion of Greece, a Greek army led by the Spartan general Pausanias captured the city, which remained an independent, yet subordinate, city under the Athenians, and later to the Spartans after 411 BC. A farsighted treaty with the emergent power of Rome in c. 150 BC, which stipulated tribute in exchange for independent status allowed it to enter Roman rule unscathed. This treaty would pay dividends retrospectively as Byzantium would maintain this independent status, and prosper under peace and stability in the Pax Romana for nearly three centuries until the late 2nd century AD.