 Complete the following phrase:  Conceived and built between 1451 and 1452 on the orders of Ottoman Sultan Mehmed II, the complex was commissioned in preparation for a planned Ottoman siege on the then-Byzantine city of Constantinople, with the goal of cutting off maritime military and logistical relief that could potentially come to the Byzantines aid by way of the Bosphorus Strait, hence the fortresss alternative name, Boğazkesen, i.e. Strait-cutter Castle. Its older sister structure, Anadoluhisari Anatolian Fortress, sits on the opposite banks of the Bosporus, and the two fortresses worked in tandem during 