with its 11 fortified gates and 192 towers, this great chain of double walls sealed Constantinople's landward side against invasion for more than a thousand years. Extending for a distance of 6.5 km (4 miles) form the sea of Marmara to the Golden Horn, the walls are built in layers of red tile alternating with limestone blocks. Different sections can be reached by metro, tram, train or bus; but to see their whole length you will need to take a taxi or dolmuş  along the main road that runs outside them.
 
The walls were built between AD 412-22, during the reign of Theodosius II . In 447 an earthquake destroyed 54 of the towers but these were immediately rebuilt, under threat of the advancing Attila the Hun. Subsequently the walls resisted sieges by Arabs, Bulgarians, Russians and Turks. Even the determined armies of the Fourth Crusade only managed to storm the ramparts along the Golden Horn, while the land walls stood firm.
Mehmet the Conqueror finally breached the walls in May 1453 . Successive Ottoman sultans then kept the walls in good repair until the end of the 17th century.
Recently, large stretches of the walls, particularly around Belgratkapi (Belgrade Gate) have been rebuilt. Byzantine scholars have criticized the restoration for insensitive use of modern building materials, but the new sections do give you an idea of how the walls used to look. Many, although not all, of the gateways are still in good repair. Mehmet the Conqueror directed his heaviest cannon at the St Romanus and Charsius gates. Under the Ottomans, the former became known as Topkapi, the Gate
of the Cannon (not to be confused with Topkapi Palace, . Unfortunately, a section of walls close to this gate was demolished in the 1950s to make way for a road, Millet Caddesi. The Charsius Gate (now called Edirnekapi), Silivrikapi, Yeni Mevlanakapi and other original gates still give access to the city. The Yedikule Gate (which stands beside the fortress of the same name) has an imperial Byzantine eagle carved above its main archway. (s1)