Rumelihisari (also known as Rumelian Castle and Roumeli Hissar Castle) is a fortress located in the Sariyer 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 Çandarli Halil Pasha, who built the big tower next to the gate, Zaganos Pasha, who built the south tower, and Sarica Pasha, who built the north tower.
Rumelihisari is situated at the narrowest point with 660 meters of the Bosphorus strait, just opposite of Anadoluhisari (Anatolian Castle) on the Anatolian side, which is another Ottoman fortress that was built between 1393 and 1394 by Sultan Bayezid I.
Rumelihisari 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 Anadoluhisari was thus well known to the Ottomans. On the location of Rumelihisari, 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.