Istanbul (Turkish: İstanbul; historically Byzantium and later Constantinople) is the largest city in Turkey, largest city proper and second largest metropolitan area in Europe, and fourth largest city proper in the world with a population of 12.6 million. Istanbul is the cultural and financial center of Turkey. The city covers 27 districts of the Istanbul province. It is located on the Bosphorus Strait and encompasses the natural harbor known as the Golden Horn, in the northwest of the country. It extends both on the Europe (Thrace) and on the Asia (Anatolia) side of the Bosphorus, and is thereby the only metropolis in the world that is situated on two continents. In its long history, Istanbul served as the capital city of the Roman Empire (330–395), the East Roman (Byzantine) Empire (395–1204 and 1261–1453), the Latin Empire (1204–1261), and the Ottoman Empire (1453–1922). The city was chosen as joint European Capital of Culture for 2010. The historic areas of Istanbul were added to the UNESCO World Heritage List in 1985.

In 2008, during the construction works of the Yenikapı subway station and the Marmaray tunnel at the historic peninsula on the European side, a previously unknown Neolithic settlement dating from circa 6500 BC has been discovered. The first human settlement on the Anatolian side, the Fikirtepe mound, is from the Copper Age period, with artifacts dating from 5500–3500 BC. In nearby Kadıköy (Chalcedon) a port settlement dating back to the Phoenicians has been discovered. Cape Moda in Chalcedon was the first location which the Greek settlers from Megara chose to colonize in 685 BC, prior to colonizing Byzantion on the European side of the Bosphorus under the command of King Byzas in 667 BC. Byzantion was established on the site of an ancient port settlement named Lygos, founded by Thracian tribes between the 13th and 11th centuries BC, along with the neighbouring Semistra, of which Plinius had mentioned in his historical accounts. Only a few walls and substructures belonging to Lygos have survived to date, near the Seraglio Point (Turkish: Sarayburnu), where the famous Topkapı Palace now stands. During the period of Byzantion, the Acropolis used to stand where the Topkapı Palace stands today.

After siding with Pescennius Niger against the victorious Roman emperor Septimius Severus, the city was besieged by the Romans and suffered extensive damage in 196 AD. Byzantium was rebuilt by Severus and quickly regained its previous prosperity, being temporarily renamed as Augusta Antonina by the emperor, in honor of his son.

The location of Byzantium attracted Constantine I in 324 after a prophetic dream was said to have identified the location of the city; but the true reason behind this prophecy was probably Constantine's final victory over Licinius at the Battle of Chrysopolis (Üsküdar) on the Bosphorus, on 18 September, 324, which ended the civil war between the Roman Co-Emperors, and brought an end to the final vestiges of the Tetrarchy system, during which Nicomedia (present-day İzmit, 100 km (62 mi) east of Istanbul) was the most senior Roman capital city. Byzantium (now renamed as Nova Roma which eventually became Constantinopolis, i.e. "The City of Constantine") was officially proclaimed the new capital of the Roman Empire six years later, in 330. Following the death of Theodosius I in 395 and the permanent partition of the Roman Empire between his two sons, Constantinople became the capital of the Eastern Roman (Byzantine) Empire. As well as being the centre of an imperial dynasty, the unique position of Constantinople at the centre of two continents made the city a magnet for international commerce, culture and diplomacy. The Byzantine Empire was distinctly Greek in culture and became the centre of Greek Orthodox Christianity, while its capital was adorned with many magnificent churches, including the Hagia Sophia, once the world's largest cathedral. The seat of the Patriarch of Constantinople, spiritual leader of the Eastern Orthodox Church, still remains in the Fener (Greek: Phanar) district of Istanbul.

In 1204, the Fourth Crusade was launched to capture Jerusalem, but had instead turned on Constantinople, which was sacked and desecrated. The city subsequently became the centre of the Catholic Latin Empire, created by the crusaders to replace the Orthodox Byzantine Empire, which was divided into a number of splinter states, of which the Empire of Nicaea was to recapture Constantinople in 1261 under the command of Michael VIII Palaeologus.

In the last decades of the Byzantine Empire, the city had decayed as the Byzantine state became increasingly isolated and financially bankrupt, its population had dwindled to some thirty or forty thousand people whilst large sections remained uninhabited. Due to the ever increasing inward turn the Byzantines took, many facets of their surrounding empire were now falling apart, leaving them vulnerable to attack. Ottoman Turks began a strategy by which they took selected towns and smaller cities over time, enveloping Bursa in 1326, Nicomedia in 1337, Gallipoli in 1354, and finally Adrianople in 1362. This essentially cut off Constantinople from its main supply routes, strangling it slowly.

On 29 May 1453, Sultan Mehmed II "the Conqueror" captured Constantinople after a 53-day siege (during which the last Roman/Byzantine emperor, Constantine XI, died near the Porta Aurea while defending the city) and proclaimed that Constantinople was now the new capital of the Ottoman Empire. Sultan Mehmed's first duty was to rejuvenate the city economically, creating the Grand Bazaar and inviting the fleeing Orthodox and Catholic inhabitants to return. Captured prisoners were freed to settle in the city whilst provincial governors in Rumelia and Anatolia were ordered to send four thousand families to settle in the city, whether Muslim, Christian or Jew, to form a unique cosmopolitan society. The Sultan also endowed the city with various architectural monuments, including the Topkapı Palace and the Eyüp Sultan Mosque. Religious foundations were established to fund the construction of grand imperial mosques (such as the Fatih Mosque which was built on the spot where the Church of the Holy Apostles once stood), adjoined by their associated schools, hospitals and public baths. Suleiman the Magnificent's reign of the Ottoman Empire from 1520 to 1566 was a period of great artistic and architectural achievements. The famous architect Sinan designed many mosques and other grand buildings in the city, while Ottoman arts of ceramics, calligraphy and miniature also flourished.

When the Republic of Turkey was founded in 1923 by Mustafa Kemal Atatürk, the capital was moved from Istanbul to Ankara. In the early years of the republic, Istanbul was overlooked in favour of the new capital. However, starting from the late 1940s and early 1950s, Istanbul underwent great structural change, as new public squares (such as Taksim Square), boulevards and avenues were constructed throughout the city; sometimes at the expense of the demolition of many historical buildings. Starting from the 1970s, the population of Istanbul began to rapidly increase, as people from Anatolia migrated to the city in order to find employment in the many new factories that were constructed at the outskirts of the sprawling metropolis. This sudden sharp rise in the city's population caused a large demand for housing development, and many previously outlying villages and forests became engulfed into the greater metropolitan area of Istanbul. Illegal construction, combined with corner-cutting methods, have accounted for the reason why 65% of the buildings in Istanbul are built without proper planning. The concerns have increased due to the serious nature of the Izmit earthquake of August 17, 1999.

Throughout its long history, Istanbul has acquired a reputation for being a cultural and ethnic melting pot. As a result, there are many historical mosques, churches, synagogues, palaces, castles and towers to visit in the city. Some of these historical structures, which draw millions to the city every year, reflect the heart and soul of Istanbul.

Ancient Greek:

The famous Maiden's (Leander's) Tower, one of the symbols of Istanbul, was originally built by the ancient Athenian general Alcibiades in 408 BC to control the movements of the Persian ships in the Bosphorus strait.

Roman:

The most important monuments of Roman architecture in the city include the Column of Constantine (Turkish: Çemberlitaş), which was erected in 330 by Constantine the Great for marking the declaration of the new capital city of the Roman Empire. The Mazulkemer Aqueduct, the Valens Aqueduct, the Column of the Goths at the Seraglio Point, the Milion which served for calculating the distances between Constantinople and other cities of the Roman Empire, and the Hippodrome of Constantinople which was built following the model of the Circus Maximus in Rome are other Roman era structures in the city. Construction of the Walls of Constantinople began under Constantine the Great, who enlarged the previously existing walls of Byzantium in order to defend the new Roman capital city which quickly grew following its proclamation as Nova Roma. A new set of walls was built further west during the reign of Theodosius II, and rebuilt after an earthquake in 447 in their current shape.

Byzantine:

The early Byzantine architecture followed the classical Roman model of domes and arches, but further improved these architectural concepts, as evidenced with the Hagia Sophia, which is the largest structure on Sultanahmet Square in the Eminönü district. The Hagia Sophia was designed by Isidorus and Anthemius as the third church to rise on this location, between 532 and 537, following the Nika riots (532) during which the second church was destroyed (the first church, known as the Megala Ekklessia ("Great Church") was inaugurated by Constantius II in 360; the second church was inaugurated by Theodosius II in 405, while the third and current one was inaugurated by Justinian in 537). The Church of Saints Sergius and Bacchus (commonly known as the Little Hagia Sophia), which was the first church built by Justinian in Constantinople and edificed between 527 and 536, had earlier signaled such an improvement in the design of domed buildings, which require complex solutions for carrying the structure. The present-day Hagia Irene (which was originally built by Constantine in the 4th century, but was later enlarged by Justinian in the 6th century) and the Basilica Cistern are also from this period.

The most important churches which were built after the Byzantines recovered Constantinople from the Latin Crusaders in 1261 include the Pammakaristos Church and Chora Church. Also in this period, the Genoese Podestà of Galata, Montano de Marinis, built the Palazzo del Comune (1316), an identical copy of the San Giorgio Palace in Genoa, which still stands in ruins on a parallel side street to the north of Bankalar Caddesi (Banks Street) in Galata, together with its adjacent buildings and numerous Genoese houses from the early 1300s. The Genoese also built the Galata Tower, which they named as Christea Turris (Tower of Christ), at the highest point of the citadel of Galata, in 1348.

Ottoman:

The Ottoman Turks built the Anadoluhisarı on the Asian side of the Bosphorus in 1394, and the Rumelihisarı at the opposite (European) shore, in 1452, a year before the conquest of Constantinople. The main purpose of these castles, armed with the long range Balyemez (Faule Metze) cannons, was to block the sea traffic of the Bosphorus and prevent the support ships from the Genoese colonies on the Black Sea ports, such as Caffa, Sinop, and Amasra, from reaching Constantinople and helping the Byzantines during the Turkish siege of the city. The first mosque on the European side of Istanbul was built inside the Rumeli Castle in 1452.

Following the Ottoman conquest of the city, Sultan Mehmed II initiated a wide scale reconstruction plan, which included the construction of grand buildings such as the Topkapı Palace, Grand Bazaar and the Yedikule (Seven Towers) Castle which guarded the main entrance gate of the city, the Porta Aurea (Golden Gate). The first grand mosque which was built in the city proper was the Eyüp Sultan Mosque in around 1459. The mosque was built on the site of the grave of Abu Ayyub al-Ansari, a companion of the Prophet Muhammad who had died outside the land walls of Constantinople (walls of Theodosius II) in 669, during the early skirmishes which preluded the Arab siege (674-678) to take the city. The first imperial mosque inside the city walls was the Fatih Mosque (1470) which was built on the site of the Church of the Holy Apostles, an important Byzantine church originally edificed in the time of Constantine the Great. Many other imperial mosques were built in the following centuries, such as the famous Süleymaniye Mosque (1557) which was ordered by Suleiman the Magnificent and designed by the great Ottoman architect Sinan, and the famous Sultan Ahmet Mosque (1616) which is also known as the Blue Mosque for the blue tiles that adorn its interior. In the centuries following Mehmed II, many new important buildings, such as the Süleymaniye Mosque, Sultanahmet Mosque, Yeni Mosque and numerous others were constructed.

In the 18th and 19th centuries, traditional Ottoman architectural styles were gradually replaced by European styles, such as the Baroque style interiors of the Aynalıkavak Palace (1677–1679) and Nuruosmaniye Mosque (1748–1755, the first Baroque style mosque in the city, also famous for its Baroque fountain), and the 18th century Baroque additions to the Harem section of the Topkapı Palace. Following the Tanzimat reforms which effectively started Turkey's Europeanization process in 1839, new palaces and mosques were built in Neoclassical, Baroque and Rococo styles, or a mixture of all three, such as the Dolmabahçe Palace, Beylerbeyi Palace and Ortaköy (Mecidiye) Mosque.

Starting from the early 19th century, the areas around İstiklal Avenue were filled with grandiose embassy buildings belonging to prominent European states, and rows of European (mostly Neoclassical and later Art Nouveau) style buildings started to appear on both flanks of the avenue. Istanbul especially became a major center of the Art Nouveau (Liberty) movement in the late 19th and early 20th centuries, with famous architects of this style like Raimondo D'Aronco building many palaces and mansions in the city proper and on the Princes' Islands. His most important works in the city include several buildings of the Yıldız Palace complex, and the Botter House on İstiklal Avenue. The famous Camondo Stairs on Bankalar Caddesi (Banks Street) in Karaköy (Galata) is also a beautiful example of Art Nouveau architecture. Other important examples are the Hıdiv Kasrı (Khedive Palace) on the Asian side of the Bosphorus, Flora Han in Sirkeci, and Frej Apartmanı in the Şişhane quarter of Beyoğlu.