The Theodosian Walls
The land walls of Constantinople, built in the fifth century and unbreached by assault until 1453.
A triple line of moat, outer wall and inner wall, six kilometres from the Sea of Marmara to the Golden Horn, held off Avars, Arabs, Bulgars and Rus for a thousand years. What finally broke them was not a better army but a better gun: the Ottoman siege train of 1453 included bombards that could throw stone shot heavy enough to bring down masonry designed for a pre-gunpowder world. Read the full story in our article on The Fall of Constantinople 1453: The End of Byzantium and Dawn of a New Era.
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