Turks had already tried to conquer Constantinople four times until Mehmet II. After becoming sultan, Mehmet II immediately built the Rumeli Fortress and restored the Anadolu Fortress in order to prevent the passage of any reinforcements through the Bosphorus.
Preparations, which took two years, included enhancing the fleet and manufacturing cannons.
In April 1453, an army of 200,000 soldiers and a fleet of 400 ships were ready in front of Constantinople. In the meantime, the Byzantines blocked the entrance of the Golden Horn by stretching chains across it. The walls of Constantinople were supported with more soldiers. The main intention of the emperor was, in case of attack, to gain time with an expectation of help from the western world.
The siege started on April 6 and continued unexpectedly. Mehmet II, to the surprise of the Byzantines, took his ships to the Golden Horn over a hill near Tophane by pulling them with animal and human power on oily wood pieces. A siege of 53 days ended on May 29, 1453. Mehmet II ceremoniously entered the city and this considerable victory gave him the title Fatih "conqueror" in the Islamic world.
The tolerance of the Ottoman Turks has meant that a majority of religious buildings from the Byzantine period still exist, if only as churches converted to mosques. Compared to many other countries where these kinds of buildings were generally destroyed, it should be noted that religious tolerance was not a new tradition in Anatolian civilizations.