[9] In 330 CE, the Roman emperor Constantine the Great made it his imperial capital, renaming it first as New Rome (Ancient Greek: Νέα Ῥώμη Nea Rhomē; Latin: Nova Roma)[10] and then finally as Constantinople (Constantinopolis) after himself.

The city played a key role in the advancement of Christianity during Roman and Byzantine times, hosting four of the first seven ecumenical councils before its transformation to an Islamic stronghold following the fall of Constantinople in 1453 CE—especially after becoming the seat of the Ottoman Caliphate in 1517.

[15] The historic centre of Istanbul is a UNESCO World Heritage Site, and the city hosts the headquarters of numerous Turkish companies, accounting for more than thirty percent of the country's economy.

[43] Byzantium's decision to side with the Roman usurper Pescennius Niger against Emperor Septimius Severus cost it dearly; by the time it surrendered at the end of 195 CE, two years of siege had left the city devastated.

[51][52] 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.

Cognizant that revitalization would fail without the repopulation of the city, Mehmed II welcomed everyone–foreigners, criminals, and runaways– showing extraordinary openness and willingness to incorporate outsiders that came to define Ottoman political culture.

[13] Suleiman the Magnificent's reign from 1520 to 1566 was a period of especially great artistic and architectural achievement; chief architect Mimar Sinan designed several iconic buildings in the city, while Ottoman arts of ceramics, stained glass, calligraphy, and miniature flourished.

[79] The civil strife and political uncertainties in the Ottoman Empire during the months after the revolution encouraged Austria-Hungary to annex Bosnia and Bulgaria to declare its independence in a jointly coordinated move on 5 October 1908.

[84] Turkish forces of the Ankara government, commanded by Şükrü Naili Pasha (3rd Corps), entered the city with a ceremony on 6 October 1923, which has been marked as the "Liberation Day of Istanbul" (İstanbul'un Kurtuluşu), and has been commemorated annually since.

[91] From the late 1940s and early 1950s, Istanbul underwent great structural change, as new public squares, boulevards, and avenues were constructed throughout the city, sometimes at the expense of historical buildings.

[92] The overall population of Istanbul began to rapidly increase in the 1970s, as people from Anatolia migrated to the city to find employment in the many new factories that were built on the outskirts of the sprawling metropolis.

This sudden, sharp rise in the city's population caused a large demand for housing, and many previously outlying villages and forests became engulfed into the metropolitan area of Istanbul as result of urban sprawl.

[128] Air pollution in Turkey is acute in İstanbul with cars, buses and taxis causing frequent urban smog,[129] as it is one of the few European cities without a low-emission zone.

[135] Galata (Karaköy) is today a quarter within the Beyoğlu district, which forms Istanbul's commercial and entertainment center and includes İstiklal Avenue and Taksim Square.

Lining both the European and Asian shores of the Bosporus are the historic yalıs, luxurious chalet mansions built by Ottoman aristocrats and elites as summer homes.

[140] However, Kozyatağı–Ataşehir, Altunizade, Kavacık and Ümraniye, all together having around 1.4 million sqm of class-A office space, are now important "edge cities", i.e. corridors and nodes of business and shopping centers and of tall residential buildings.

[139] As a result of Istanbul's exponential growth in the 20th century, a significant portion of the city is composed of gecekondus (literally "built overnight"), referring to illegally constructed squatter buildings.

The Neolithic settlement in the Yenikapı quarter on the European side, which dates back to c. 6500 BCE and predates the formation of the Bosporus by approximately a millennium, when the Sea of Marmara was still a lake,[citation needed] was discovered during the construction of the Marmaray railway tunnel.

[151] Built of red granite, 31 m (100 ft) high, it came from the Temple of Karnak in Luxor, and was erected there by Pharaoh Thutmose III (r. 1479 – 1425 BCE) to the south of the seventh pylon.

Over the next four centuries, the Ottomans transformed Istanbul's urban landscape with a vast building scheme that included the construction of towering mosques and ornate palaces.

[169] Small settlements adjacent to major population centers in Turkey, including Istanbul, were merged into their respective primary cities during the early 1980s, resulting in metropolitan municipalities.

The Municipal Council of Istanbul is responsible for citywide issues, including managing the budget, maintaining civic infrastructure, and overseeing museums and major cultural centers.

[210] The majority of the Catholic Levantines (Turkish: Levanten) in Istanbul and İzmir are the descendants of traders/colonists from the Italian maritime republics of the Mediterranean (especially Genoa and Venice) and France, who obtained special rights and privileges called the Capitulations from the Ottoman sultans in the 16th century.

[222] For Ekrem İmamoğlu, winning the mayoralty of Istanbul was a huge moral victory, but for Erdoğan it had practical ramifications: His party, AKP, lost control of the $4.8 billion municipal budget, which had sustained patronage at the point of delivery of many public services for 25 years.

The Bosporus, providing the only passage from the Black Sea to the Mediterranean, is the world's busiest and narrowest strait used for international navigation, with more than 200 million tons of oil passing through it each year.

[229] International conventions guarantee passage between the Black and the Mediterranean seas,[230] even when tankers carry oil, natural gas, chemicals, and other flammable or explosive materials as cargo.

[274] Since 2015, all types of parades at Taksim Square and İstiklal Avenue (where, in 2013, the Gezi Park protests took place) have been denied permission by the AKP government, citing security concerns, but hundreds of people have defied the ban each year.

The TVF Burhan Felek Sport Hall is one of the major volleyball arenas in the city and hosts clubs such as Eczacıbaşı VitrA, Vakıfbank SK, and Fenerbahçe who have won numerous European and World Championship titles.

[343] Istanbul's local public transportation system is a network of commuter rail, trams, funiculars, metro lines, buses, bus rapid transit, and ferries.

[76] Regular service to Bucharest and Thessaloniki continued until the early 2010s, when the former was interrupted for Marmaray construction but started running again in 2019 and the latter was halted due to economic problems in Greece.