Rumelihisarı Rumelihisarı ( also known as Rumelian Castle and Roumeli Hissar 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 . Construction . 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 Pasha s 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 . The sultan wanted to cheer up the builders so he allegedly ordered them to build the castle in the shape of the name of Muhammad the Muslim prophet , which can be seen from above . Muhammad and Mehmed share the same Arabic spelling ( محمد ) , and so he may have also made the fortress as an homage to himself . Architecture . The Rumelihisarı fortification has one small tower , three main towers , and thirteen small watchtower s placed on the walls connecting the main towers . One watchtower is in the form of a quadrangular prism , six watchtowers are shaped as prisms with multiple corners and six others are cylindrical . The main tower in the north , the Saruca Pasha Tower , is in cylindrical form with its 9 stories and height of , has a diameter of and its walls are thick . Today , this tower is called the " Fatih ( Conqueror ) Tower " after Sultan Mehmed II . Halil Pasha Tower , a dodecagon prism , which stands at the waterfront in the middle of the fortress , has also 9 stories . It is high with a diameter and the walls are thick . The main tower in the south , the Zağanos Pasha Tower , has only 8 stories . The cylindrical tower is high , has a diameter with thick walls . The space within each tower was divided up with wooden floors , each equipped with a furnace . Conical wooden roofs covered with lead crowned the towers . The outer curtain walls of the fortress are from north to south long and from east to west varying between long . Its total area is . The fortress had three main gates next to the main towers , one side gate and two secret gates for the arsenal and food cellars next to the southern tower . There were wooden houses for the soldiers and a small mosque , endowed by the Sultan at the time of construction . Only the minaret shaft remains of the original mosque , while the small masjid added in the mid - 16th century has not survived . Water was supplied to the fortress from a large cistern underneath the mosque and distributed through three wall - fountains , of which only one has remained . Two inscriptive plaques are found attached on the walls . The fortress , designed by architect Müslihiddin , was initially called " Boğazkesen " , literally meaning " The Strait Cutter " , referring to the Bosporus Strait . The name carries a secondary and more macabre meaning ; as " boğaz " not only means " strait " but also " throat " in Turkish . It was later renamed as Rumelihisarı , which means " Fortress on the Land of the Romans " , i . e . Byzantine Europe , or the Balkan peninsula . Usage in the past . A battalion of 400 Janissaries were stationed in the fortress , and large cannons were placed in the Halil Pasha Tower , the main tower on the waterfront . A Venetian ship coming from the Black Sea which ignored the order to halt by the commander of the fortress , Firuz Ağa , was bombarded and sunk , and its surviving crewmen were beheaded as a warning to any who might attempt the same . These cannons were later used until the second half of the 19th century to greet the sultan when he passed by sea . After the fall of Constantinople , the fortress served as a customs checkpoint . Rumelihisarı , which was designated to control the passage of ships through the strait , eventually lost its strategic importance when a second pair of fortresses was built further up the Bosphorus , where the strait meets the Black Sea . In the 17th century , it was used as a prison , primarily for foreign prisoners of war . Rumelihisarı was partly destroyed by an earthquake in 1509 , but was repaired soon after . In 1746 , a fire destroyed all the wooden parts in two of the main towers . The fortress was repaired by Sultan Selim III ( 1761 – 1807 ) . However , a new residential neighborhood was formed inside the fortress after it was abandoned in the 19th century . Today . In 1953 , on the orders of President Celal Bayar , the inhabitants were relocated and extensive restoration work began on 16 May 1955 , which lasted until 29 May 1958 . Since 1960 Rumelihisarı has been a museum and an open - air theater for various concerts at festivals during the summer months . The Fatih Sultan Mehmet Bridge which spans the Bosporus is located close to the fortress , to the north . Rumelihisarı is open to public every day except Wednesdays from 9 : 00 to 16 : 30 . The fortress was depicted on various Turkish banknotes during 1939 - 1986 .