<P> Initially built by Constantine the Great , the walls surrounded the new city on all sides , protecting it against attack from both sea and land . As the city grew , the famous double line of the Theodosian Walls was built in the 5th century . Although the other sections of the walls were less elaborate , when well - manned , they were almost impregnable for any medieval besieger , saving the city , and the Byzantine Empire with it , during sieges from the Avar - Sasanian coalition , Arabs , Rus ' , and Bulgars , among others . The advent of gunpowder siege cannons rendered the fortifications vulnerable , but cannon technology was not sufficiently advanced to capture the city on its own , and the walls could be repaired between reloading . Ultimately the city fell from sheer weight of numbers of the Ottoman forces on 29 May 1453 after a six - week siege . </P>
who constructed the massive defensive wall around constantinople in the early 5th century ce
Constantine the Great