Rumelihisarı, also known as Rumeli Fortress, is a historic 15th-century fortress located on the European side of the Bosphorus in Istanbul, Turkey. Built by Sultan Mehmed II in just over four months in 1452, it played a crucial role in the Ottoman siege of Constantinople, controlling naval traffic on the Bosphorus and preventing outside aid from reaching the city. The fortress features three imposing towers connected by thick walls and offers stunning views of the Bosphorus Strait.