The chain across the Golden Horn 

The Golden Horn is a deep inlet running north-west off the Bosphorus, and it is the reason Constantinople existed. It gave the city a sheltered, ice-free anchorage with a working depth that medieval fleets could use in any weather. It also gave the city its softest flank, because the walls along the Horn were single and lower than the great land walls, built on the assumption that no hostile fleet would ever get inside.

That assumption was enforced by a chain. A boom of heavy iron links, floated on wooden barrels or logs and anchored at one end near the city and at the other under the Genoese colony at Galata on the far shore, could be winched taut across the mouth of the inlet. Sections of the chain survive in Istanbul and the links are the size of a forearm. It did not need to stop a ship dead. It needed only to hold a hull in place under the walls long enough for the defenders to sink it, and every captain knew that.

In 1453 the chain did its job. Behind it sat the defending fleet, a modest force of Byzantine, Genoese and Venetian vessels, safe from the much larger Ottoman navy in the Bosphorus. That security mattered more than the ship count suggests. It meant the defenders could leave the Horn wall thinly held and concentrate their few thousand men on the land walls, where the guns were. In a siege where the whole garrison numbered around seven thousand against a besieging army many times larger, every stretch of wall that could be left quiet was a gift.

The chain also produced the siege's most cheerful episode for the defence. In late April a small squadron of Genoese and imperial ships fought its way through the Ottoman fleet in the open water outside the boom, in full view of both armies and of the sultan on the shore, and reached the safety of the Horn. It was a tactical success and a morale event. It also convinced Mehmed that his navy could not force the entrance, which pushed him towards a solution nobody in the city was defending against.

There is a persistent story that the chain was cut or broken. It was not. It was still in place when the city fell, and it was later hauled up and kept as a trophy. This is the pattern worth carrying out of the chapter. A defence that cannot be beaten on its own terms does not survive by being unbeatable. It survives only until the attacker stops accepting those terms.

Mehmed's answer was to move his ships where the water was not.