Constantinople’s great city walls were breached onlytwice in 1600 years.
(Fortress of 7 Towers)
You can admire them at many points in Istanbul(especially the several gates in the walls), but it’s easiest to combine a view of the walls with a visit to the Kariye Museum (Chora Church) and the nearby Byzantine palace of Tekfur Saray (Palace of Constantine Porphyrogenetus) in the Edirnekapı (Edirne Gate) district.
The first breach of the walls was by the Fourth Crusadein 1204, the second by the cannons and troops of Sultan Mehmet the Conqueror in 1453.
Several of the main gates were restored during the 1990s, but other parts of the walls have been left in their unrestored state.