Rumeli Fortress (Rumelian Castle) is located at the narrowest point of the Bosphorus. It was built by 7th Ottoman Sultan Mehmet II (conqueror) in 1452. Before the conquest of Istanbul, the sultan and the grand viziers had decided to control the sea traffic at the Bosphorus. Therefore, the Ottomans built this fortress on the European side where is located opposite of the other fortress on the Asian side (Anatolian Fortress-1394). They built Rumeli Fortress in 4 months. The Ottomans conquered Istanbul in 1453.
Rumeli Fortress has lots of small towers, and each has a different height. Moreover, the Ottomans built the first mosque of Istanbul in this fortress before the conquest. The Ottomans had used it as a castle to control the sea traffic for centuries. After many renovations, the castle has been used as an open-air concert, opera, and theatre since 1951. Today, Rumeli Fortress is an open-air museum also. The fortress has a beautiful view of the Bosphorus, and it is worth visiting.