In the stranglehold that eventually throttled Constantinople in 1453, the Rumeli Hisar (castle) served as the lynch pin. Constructed on the narrowest point of the Bosphorus north of the city, it blocked Byzantine access to the Black Sea. Today, it’s an iconic monument surrounded by posh neighborhoods and waterside brunch restaurants. Easily accessible, the size allows visitors to spread out in the fortress and climb the high walls and towers, where views extend all the way to the Black Sea on clear days. A good sense of balance is required.