Rumeli Fortress (also known as Rumeli Hisarı in Turkish) is a historical fortress located on the European side of Istanbul, Turkey. The fortress was built in 1452 by the Ottoman Sultan Mehmed II, also known as Mehmed the Conqueror, in preparation for his conquest of Constantinople (now Istanbul) the following year.

Rumeli Fortress is strategically located at the narrowest point of the Bosphorus Strait, where it is only 660 meters wide. This location allowed the Ottomans to control the maritime traffic passing through the strait, and thus prevent any attempts by the Byzantines or other potential invaders to reach Constantinople by sea.

The fortress is a large, triangular structure made of stone and brick, with three main towers and thirteen smaller towers. It also features a large courtyard, a mosque, and several other buildings. The fortress walls are over 6 meters thick and up to 30 meters high in some places, making it a formidable defensive structure.

Today, Rumeli Fortress is open to the public as a museum, and visitors can explore the fortress walls and towers, as well as visit the museum exhibits and learn more about the fortress's history and significance. It is a popular tourist attraction in Istanbul, offering panoramic views of the Bosphorus and the surrounding areas.