The greatest defensive element of the city of Constantinople were its walls, called the Theodosian Walls after the Emperor Theodosios, during whose reign the walls were built. Although named for Theodosius, he was still a boy when construction was completed by his then-regent, Anthemius, in 413.
The walls stretched 6 and a half km in length, running from the Sea of Marmara in the south, to the Golden Horn in the north. The city’s main road, the Mese, started at the Milion near the Hagia Sophia, the Great Palace, and Hippodrome, and exited the city near the most southern point at the Porta Aurea – the Golden Gate, later called the Yedikule Gate by the Turks.
The wall had several layers of defense, including an low outer wall, a moat, and a higher inner wall. The walls still stand, after more than 16 centuries, but they show their age. Its many towers are empty now, although you can still see in their inner walls (as above) where the builders left niches to lay beams on which they built floors.
The walls still stand, although Istanbul has grown far beyond its boundaries. The Porta Aurea is almost the worst part of it and no longer easily accessible. The Byzantium 1200 website has a nice virtual recreation of the walls and the Porta Aurea, as well as a Youtube video that includes the walls. I highly recommend viewing it to get a feel for how the walls would have looked in their heyday.
This tower was close to the Porta Aurea and appears to have been relatively well maintained. There have been efforts to rebuild/repair the walls in recent years, but according to our guide, the effort was badly done and the reconstruction stopped.
Many of the old gates (of which there were quite a few) are now used for cars, as this one is. Otherwise, you can find graffiti, a few homeless people nesting, and the occasional tourist visiting them.
Just outside the walls our guide showed us these old houses, which are examples of the houses built during the Ottoman period. The way the upper stories hang over the road (which seems to have been common in medieval Europe), they made me wonder if they were similar to houses built during the Byzantine period.