These walls were almost inpenetrable, saving the city during sieges from the Huns, Bulgars, Arabs, and Rus', among others.
Theodosian Walls
Constantinople's walls were only breached twice, once in 1204 by the armies of the Fourth Crusade, and then in 1453 - a thousand years after they were built - by the gigantic cannon of Sultan Mehmet the Conqueror.