The vast majority of visitors to İstanbul pay little heed to the city's most extensive Byzantine remains -- the land walls. These formidable fortifications were built in the fifth century A.D. to protect Constantinople, the booming capital of the mighty Byzantine Empire, from attack by armies approaching from the west.

The walls did their job -- and were only breached twice: once by the Crusaders in the 13th century and then by Mehmet the Conqueror in 1453. Today's invaders, tourists, pass through at will. Most may idly glance at the walls as they make their way into the city from Atatürk Airport by metro, tram, bus or taxi. But very few, besotted as they are by the wonders of Sultanahmet or the fleshpots of Beyoğlu, see the walls again until they are whisked through them on their way back to the airport. This is a great shame. Stretching for around six kilometers from the Sea of Marmara in the south to the Golden Horn in the north, 13 meters high and five meters thick, pierced by 10 major gateways and surmounted by numerous watchtowers, they are a superb example of Byzantine defensive architecture.

The best way to appreciate the walls is on foot. Those of you who are good at math will have already worked out that at an average walking speed of three kilometers per hour it will only take a couple of hours to walk their six-kilometer length. Think again! To get the most out of the experience, it's best to allow a whole day. There is much to be seen en route -- and a great lunch stop. The easiest way to get to the start point is by suburban train from Sirkeci or Cankurtan stations to Yedikule. From the station, cross the busy coast road to the Marble Tower, situated right on the Sea of Marmara. This 30-meter high structure, its base clad with marble, is thought to have been a seaside retreat for Byzantine emperors. Now head north and inland, passing the Gate of Jesus, so-called because the letters "XP," inscribed in the stonework above the entrance, form the first two letters of Christ's name. Then re-cross the coastal highway and railway line and continue north towards Yedikule.

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