The last emperor at the Kerkoporta 

The final assault began in darkness on 29 May 1453 and was organised in waves, with the least experienced troops sent first to exhaust the defence and the elite janissaries held for last. It concentrated where the guns had worked longest, on the ruined stretch in the Lycus valley, and it was fought across earth barricades rather than stone battlements.

Two things went wrong for the defence within a short span. Giovanni Giustiniani Longo, the Genoese professional who had run the wall defence for the whole siege and whose competence had held the barricades together, was badly wounded and carried away to his ship. His men, seeing him go, began to leave the line, and the Greek and Venetian units around them lost the cohesion that had survived seven weeks. Separately, Ottoman troops found their way through the Kerkoporta, a small postern in the northern section near the Blachernae palace, which had been opened for sorties and, according to the accounts, was not properly secured behind the last party to use it.

It is worth being careful here. The Kerkoporta appears in Doukas and in other contemporary or near-contemporary writers, and the details differ. Whether the gate was left open by carelessness, by panic or by ordinary confusion is not recoverable, and the later idea that a single unlocked door lost the empire is a story shaped by hindsight. The wall had been broken by artillery for weeks and the defenders were outnumbered several times over. The postern accelerated a collapse that the bombardment had already made likely.

Constantine XI Palaiologos, the eleventh emperor of that name and the last Roman emperor, was at the breach. The reliable accounts agree that he refused to flee, that he went into the fighting near the Saint Romanus sector, and that he was killed there. His body was never identified with certainty. That absence produced the legend of the marble emperor, sleeping until the day the city returns, which tells you more about what the loss meant to Greek memory than about 1453.

By the afternoon Mehmed, now Fatih, the Conqueror, entered the city and rode to Hagia Sophia. The Roman state, which had counted its emperors in an unbroken line of institutions from Augustus, ended in a single morning. The city that replaced it became the Ottoman capital and remained one of the great cities of the world.

The people who could get out were already sailing west, and many of them were carrying books.