Rumeli Fortress (Rumeli Hisarı in Turkish) is a historic castle located on the European side of Istanbul, Turkey. It was built by the Ottoman Sultan Mehmed II in the 15th century as part of his preparations for the conquest of Constantinople (Istanbul). The fortress is situated at the narrowest point of the Bosphorus Strait, which connects the Black Sea to the Marmara Sea, and it is a popular tourist attraction in Istanbul.

The fortress consists of three main towers, several smaller towers, and walls that surround an inner courtyard. The walls are approximately 250 meters long and up to 30 meters high, and they are reinforced with 13 massive towers. The castle has a rectangular shape, and it is built of stone and brick.

Rumeli Fortress was strategically important during the Ottoman period, as it controlled the Bosphorus Strait and prevented foreign ships from entering the sea of Marmara without permission. Today, the fortress is open to visitors, and it offers stunning views of the Bosphorus Strait and the city of Istanbul. Visitors can explore the castle walls, towers, and inner courtyard, and learn about the history of the fortress through exhibitions and guided tours.