While not as impressive a feat as the Theodosian defenses, the Sea Walls, originally built by order of Constantine and extended by Theodosius, are worth a mention. Sadly, there's very little left of the original section along the southern coastal road, because in 1870, railway engineers simply knocked it down to make way for the commuter train. So walls that endured 1,500 years of sieges, earthquakes, and even tsunamis fell to the onward march of urban "improvement."
The crumbling (albeit highly photogenic) decay of the sea walls greets you as you enter the Old City along the coastal road from the airport. The walls are predominantly to your left, leaving you to imagine the waves of the Marmara Sea lapping up over the road you're driving in on. Portions of the wall prop up houses constructed on the hill, and if you know what you're looking for, you can spot a handful of crumbling palatial arches remaining from the grand Bucoleon Palace. The wall was punctuated by eight gates designed to allow ships entry into the inner harbors. The Porta de Condoskali, or the Shallow Harbor Gate, now provides land access to the appropriately named neighborhood of Kumkapi (Sand Gate), now a lively area of fish restaurants recalling (barely) the fishermen's harbor that it once was. The Ahirkapi, or Stable Gate, served as the imperial port, but fell into disuse with the construction of the Bucoleon Palace to the west.