The Wall of Constantinople are an unique architectonic construction of defensive significance. Their construction began as early as in the times of ancient Greece, when Byzantium was founded by Greek colonists. The fortifications were extended by the succeeding rulers of the city during the times of the Roman Empire, and subsequently of the Byzantine Empire. After Constantinople had conquered by the Ottoman Turks they continued the walls development. The walls construction lasted from the ancient times to the 15th century. Their constant enlargement and strengthening bore fruit in the form of the world’s most sophisticated system of defensive walls. The walls survived until the 19th century, when the city expanded and started developing outside the walls, which were used as a source of building material. Nevertheless, since 1989 preservationists have been working in order to preserve the remnants of the walls.