This is a remnant of the old Galata wall. Galata was another walled city - a Genovese colony - just across the Golden Horn from Constantinople. Ostensibly neutral during the Ottoman siege of Constantinople, the Byzantine defenders of Constantinople closed the Golden Horn with a large chain, anchoring one end within the walls of Galata, forcing the Ottomans to haul their ships overland to get them into the waters of the Horn.
