The wall that held for a thousand years 

Constantinople sat on a triangle of land with water on two sides. The Sea of Marmara guarded the south, the Golden Horn the north, and both were patrolled and walled. That left one approach, a stretch of roughly six kilometres across the neck of the peninsula, and it is there that the empire spent its engineering budget.

The land walls we mean when we say the walls of Constantinople were raised in the early fifth century under the emperor Theodosius II, whose praetorian prefect Anthemius did the organising while the emperor was still a boy. They were not one wall. Coming from the west, an attacker met a broad ditch that could be flooded, then a low breastwork, then an outer wall around eight metres high studded with its own towers, then a terrace, then the inner wall, twelve metres or so of stone and brick banded together, carrying ninety-six great towers.

The genius of that arrangement is that it is defence in depth rather than defence in height. A besieger who scrambled into the ditch was under fire from two levels at once. A team that breached the outer wall found itself in a narrow killing corridor overlooked by the inner towers, with no room to bring up ladders or shelters. Every gain cost more than the last, and each gain was reversible, because the defenders could seal a gate and drop back to the next line without losing the city.

It worked with monotonous reliability. Avars and Persians in 626, Arab armies in the 670s and again in 717 and 718, Bulgars, Rus raiders, Ottoman attempts in 1394 and 1422: all of them either withdrew or turned to blockade and starvation. The one time the walls were taken, in 1204, it was the sea wall on the Golden Horn that gave way to the Fourth Crusade, not the land defences. The reputation of the fortification became a strategic fact of its own. Armies planned routes around Constantinople the way modern shipping plans around a closed canal.

There was a hidden cost to that success. The walls invited an empire to shrink behind them. By the fifteenth century the population inside had fallen to perhaps fifty thousand in a circuit built for many times that number, with fields and orchards growing in the empty quarters. Maintenance depended on a treasury that no longer existed, and manning the full six kilometres properly needed an army the city could not raise.

The walls were still unbeaten. They were simply designed against a world of rams, ladders and mangonels, and in the workshops of Europe someone was already casting a different kind of argument.