Rising from an island 250m offshore, impossibly romantic Kızkalesi Castle (also called the Sea Castle) is like a suspended dream. Check out the mosaics of birds and trees in the central courtyard, where there are the remains of two chapels side by side, and the vaulted gallery with 13 arches. Walk along the castle walls and climb one of the four towers (the square one at the southeast corner has the best views).
The castle was probably built by the Byzantines sometime after the First Crusade (1095–1099) and was extensively rebuilt in the 13th century by the Armenian King Leo I. It's possible to swim to the castle, but most people catch the boat (₺15 return) from the beach pier near Corycus Castle. Another option is to rent a dolphin- or ducked-themed pedalo (₺15) available near the pier and paddle on over.