The sea walls of Constantinople were an important component in the defensive structure of Constantinople, allowing the city to be fully enveloped in fortifications:
“The origins of the sea walls are less clear, and uncertainties remain as to their extent (or very existence) until the late seventh century. The seaward defences were a secondary concern as long as the Mediterranean remained a Roman lake. In the same year that a Vandal fleet captured Carthage (439), however, the first efforts to fortify the city’s shoreline are attested in an edict calling for the sea walls to be carried as far as the new land walls to form a complete circuit. Built on a simpler scale but at much greater length, the fortifications along the Sea of Marmara and the Golden Horn, contoured to the city’s littoral and harbours, consisted of a single wall punctuated by as many as 188 towers along the Sea of Marmora and 110 along the Golden Horn.”
“It would appear that the initial defences along the Golden Horn were incomplete or perceived as the weakest section in the entire circuit, since at the height of the siege in 626 the Avars’ Slav allies attempted a landing on the shoreline near the Blachernai quarter that was repulsed by a Byzantine flotilla assigned to the defence of that particular section. The strength of the Theodosian Walls compelled attackers to try their luck against the seemingly weaker fortifications in the Blachernai section. Other besiegers, such as the rebel Thomas the Slav in 821, the Bulgar khan Symeon in 921, the rebel Leo Tornikios in 1047, and the men of the Fourth Crusade in 1203–4 also marshalled their forces opposite the Blachernai–Golden Horn juncture of the land and sea walls.” The sea walls certainly were far weaker if an enemy fleet could get past the chain into the Golden Horn harbor, where waters were calm enough for ships to attack the walls.
“The improvement or repair of the sea walls receive attention in the sources when the threat of naval attack appeared imminent. Faced with the rise of Arab sea power in the latter seventh century, a series of emperors, particularly Tiberios III (698–705) and Anastasios II (713–15), are reported to have strengthened the sea wall in anticipation of the armada which eventually arrived in 717 and carried on a siege for over a year. This is also the first time we hear of the famous chain stretched across the mouth of the Golden Horn to deny enemy vessels access to that critical waterway. Following the near-run victory over the afore- mentioned Thomas the Slav, whose fleet broke into the Golden Horn and enabled him to make a dangerous assault on the Blachernai quarter, the emperors Michael II (820–29) and Theophilos (829–42) embarked on an ambitious programme to reinforce the sea walls, especially the vulnerable section along the Golden Horn. As was the case with the civic restoration undertaken by Constantine V (741–75), made necessary by a succession of calamities (earthquakes, plague, civil unrest), the steps taken by Michael II and Theophilos were intended both to make the capital more secure after the Arab capture of Crete, and to restore the harbour facilities along the Golden Horn which had long since fallen into decay.” Michael VIII also restored the sea walls when he was rebuilding Constantinople after 1261.
The sea walls were not needed often, but when they were they were a crucial element on the defenses of Constantinople.
Sources:
The Cambridge Companion to Constantinople – Chapter 7 The Defense of Constantinople by Eric McGreer.
shadowsofconstantinople.com 