Anadoluhisarı, known historically as Güzelce Hisar (“the Beauteous Castle”) is a medieval fortress located in Istanbul, Turkey on the Anatolian (Asian) side of the Bosporus. The complex is the oldest surviving Turkish architectural structure built in Istanbul, and further gives its name to the neighborhood around it in the city’s Beykoz district.
Anadoluhisarı was built between 1393 and 1394 on the commission of the Ottoman Sultan Bayezid I, as part of his preparations for a siege on the then-Byzantine city of Constantinople. Constructed on an area of 7,000 square metres (1.7 acres), the fortress is situated at the narrowmost point of the Bosporus, where the strait is a mere 660 meters (2,170 ft) wide. The site is bound by Göksu creek to the south, and was previously home to the ruins of a Roman temple dedicated to Uranus. Erected primarily as a watch fort, the citadel has a 25 meters (82 ft) tall, quadratic main tower within the walls of an irregular pentagon, with five watchtowers at the corners.