The first inhabitants of Istanbul date back to second millennium BC, they settled on the Asian side of the city. The first name of Istanbul comes from Megara king Byzas who took his colonists here in the 7th century BC to establish a colony named Byzantium, the Greek name for a city on the Bosphorus. Byzas decided to settle in that spot after consulting an oracle of Delphi who told him to settle across from the "land of the blind". Indeed, Byzas believed that earlier settlers must have been "blind" for not seeing this superb location at the entrance of the Bosphorus strait, only access to the Black Sea.
In the 6th century BC, The Persian Empire ruled the city and than Alexander the Great conquered it after 4th century BC, which was a peaceful period until the 2nd century BC. In 193 AD Roman Emperor Septimus Severus conquered the city and it remained under the Roman Empire rule until 4th century AD, when emperor Constantine the Great made Byzantium the capital of entire Roman Empire and gave it his name: Constantinople, and Eastern Roman Empire was called Byzantine Empire after 5th century. The city was built on seven hills, like Rome. Early Byzantine Emperors filled their city with the treasures of the ancient world, especially between 4th and 6th centuries with a population exceeding half a million.
In 532 AD, during the reign of Justinian I, riots destroyed the city but they rebuilt the city. Istanbul was besieged by the Arabs in the 7th and 8th century AD and Fourth Crusade between 1204-1261 AD destroyed and sacked all the wealth. After this, Constantinople did not regain its former richness and strength.
Ottoman Turks led by Sultan Mehmet II conquered the city of Constantine in 1453. Renamed Islambol, it became the capital city of the Ottoman Empire. In 15th and 16th centuries, the Ottoman Sultans built many mosques and public buildings, topping the population again around half million by the mid 1500's, Istanbul was a major cultural, political, and commercial center. The name of Istanbul is derived from a combination of "Islambol" ("city of Islam" in Turkish) and "eis tin Polin" ("to the City" in Greek). The Ottoman monarchy continued until 1923 when Ataturk set up the new democratic and secular Turkish Republic and made many reforms in many different fields. The capital city of Turkey is now Ankara but Istanbul still remains to be the center of commercial and the most important city of Turkey.