 The seven hills, all located in the area within the walls, first appeared when the valleys of the Golden Horn and the Bosphorus were opened up during the Secondary and Tertiary periods.In the Ottoman Age, as in the earlier Byzantine period, each hill was surmounted by monumental religious buildings (churches under the Byzantines, imperial mosques under the Ottomans).The first hill on which the ancient city of Byzantium was founded, begins from Seraglio Point and extends over the whole area containing Hagia Sophia, the Sultan Ahmed Mosque and Topkapı Palace.