Rumeli Fortress Istanbul — Bosphorus Castle

In 1452, Sultan Mehmed II ordered the construction of Rumeli Hisarı at the narrowest point of the Bosphorus (about 660 meters wide) to cut off Byzantine Constantinople from Black Sea reinforcements and grain supplies, which is why it was also called Boğazkesen, the "strait-cutter". Work began on 15 April 1452, a year before the final siege of the city. Together with Anadolu Hisarı on the Asian shore, built by Sultan Bayezid I in 1393–1394, the two fortresses let the Ottomans control the strait.

The fortress covers 31,250 square meters and has three main towers (each named after the vizier who supervised its construction), one small tower and 13 watchtowers on the connecting walls; the walls of the north tower are 7 meters thick. The three cylindrical towers are connected by curtain walls that follow the contours of the hillside. The design was purely functional — cannons placed within the towers could hit any ship attempting to pass through the strait without permission. After the conquest of Constantinople in 1453, the fortress served as a customs checkpoint and an occasional prison.