Rumeli Hisarı, built in 1452 by Sultan Mehmed II, is a massive fortress on the European side of the Bosphorus, strategically placed to control naval passage before the conquest of Constantinople. With its three large towers, thick stone walls, and steep staircases, it remains one of Istanbul’s most striking military structures. Once used as a customs checkpoint and prison, it is currently under restoration and will continue to serve as a museum after the restauration. Surrounded by lively cafés and scenic promenades, Rumeli Hisarı stands as a powerful reminder of Ottoman military strategy and history.