The land walls built by Theodosius II in the 5th century ran from the Sea of Marmara to the Golden Horn -- 6.5 kilometres of double walls and 96 towers that kept Constantinople safe for over a thousand years. Large sections survive, and a walk along them is one of the most atmospheric things you can do in Istanbul. The Yedikule (Seven Towers) fortification at the southern end is the most impressive stretch.
Theodosian Walls
Massive fifth-century double walls that defended Constantinople