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Rumelihisarı (Rumelian Castle) is a fortress located in the Sarıyer district of Istanbul, Turkey, on a hill at the European side of the Bosphorus. It gives the name of the quarter around it. It was built by the Ottoman Sultan Mehmed II between 1451 and 1452, before he conquered Constantinople. The three great towers were named after three of Mehmed II's viziers, Sadrazam Çandarlı Halil Pasha, who built the big tower next to the gate, Zağanos Pasha, who built the south tower, and Sarıca Pasha, who built the north tower.

Rumelihisarı is situated at the narrowest point with 660 meters of the Bosphorus strait, just opposite of Anadoluhisarı (Anatolian Castle) on the Anatolian side, which is another Ottoman fortress that was built between 1393 and 1394 by Sultan Bayezid I.

Rumelihisarı was built by Sultan Mehmed II between 1451 and 1452 in order to control the sea traffic on the Bosphorus strait and prevent aid from the Black Sea to reach Constantinople during the Turkish siege of the city in 1453, particularly from the Genoese colonies such as Caffa, Sinop and Amasra. In a previous Ottoman attempt to conquer the city, Sultan Murad II (1404–1451) had encountered difficulties due to a blockade of the Bosphorus by the Byzantine fleet. The necessity of a fortress opposite of Anadoluhisarı was thus well known to the Ottomans. On the location of Rumelihisarı, there had been a Roman fortification in the past, which was used as a prison by the Byzantines and Genoese. Later on, a monastery was built there.

In preparation for the conquest of Constantinople, Sultan Mehmed II (1432–1481), son of Murad II, started to realize the construction of the fortress immediately following his second ascent to the throne in 1451. He refused the plea for peace of the Byzantine Emperor Constantine XI (1404–1453), who understood the intention of the Sultan. The construction began on 15 April 1452. Each one of the three main towers was named after the Pashas who supervised their construction. The Sultan personally inspected the activities on the site. With the help of thousands of masons and workers, the fortress was completed in a record time of 4 months and 16 days on 31 August 1452.