29th May 1453, as FDR might have said, "a date which will live in infamy". The Roman Empire ends (although it has been something more and less than 'Roman' for centuries at this point).
Around three hours before dawn the final assault of the 53 day siege began. The first wave were irregulars and Azap conscripts. Cannon fodder in other words with little or no protection from the iron greeting that spat from the crossbows of the defenders as they charged the filled-in ditch of the Fosse. No official casualty figures were made but we can estimate thousands fell in that initial rush. Sultan Mehmed hardly cared, the purpose was to exhaust the defenders. Next he sent better trained and armed troops - his Sipahi. These fared little better. One must feel a degree of sorrow for the Sipahi, they were cavalry troops and no very suited to what they were being asked to do here. The final attack, several hours into the assault, were the elite Janissary. Even with this unceasing pressure the defenders appear to have not buckled until their General, Giovanni Giustiniani Longo, was struck by a bullet that penetrated his armour. Mortally wounded he told his adjutant John Dalmata to unlock the postern and get him to his ship.
It's important to note that most of the hand to hand combat has occurring in a very narrow section of the three mile wall called the Mesotechion. This area had seen the focus of the cannon barrage and the walls there were reduced to basically rubble and a hastily built stockade of barrels and anything else the desperate defenders could arrange. Prior to this assault, the Genoese mercenary troops and the Emperor's elite bodyguard had taken position in this area and locked the postern gates to their rear. Thus making the psychological statement of no retreat, stand or die. Giustiniani's action betrayed that mindset and triggered a complete collapse. Seeing their leader being carried from the field, the Genoese (who made up the majority of armoured troops) bolted after him. A stampede crushed several in the narrow postern gateway and this area became a massacre.
Tradition has it that this was the moment that Janissaries in another section of the wall (close to the Blachernae palace section where the double walls become a single set) discovered a postern (the Kerkoporta) had been left open (deliberately or not). As morale in the Mesotechion collapsed, the first Turk banners mounted the towers at the Kerkoporta and the Emperor, seeing the day lost, threw himself onto the swords of the Janissary as they broke the last resistance at the Mesotechion stockade.
From there the sack of the city began. The surviving Genoese and Venetians fled to their boats and some managed to escape, including the mortally wounded Giustiniani. They were able to do so largely because the Ottoman navy had abandoned its blockade to join in the looting of the city.
Under Islamic laws of conquest, soldiers were entitled to three days of unchecked looting (ie. the time between the fall and the fourth dawn) when a city fell in such a way. Even had he wished to, Mehmed could not have allowed things differently but it should be noted that he did specify that the buildings of the city were to be his prize and were not to be burned. Thus the sack of Constantinople in 1453 was probably less destructive than the prior one by Crusaders in 1204. Legend has it that Mehmed found a disobedient soldier hacking at a mosaic as he arrived at Hagia Sophia and had the man promptly hung.
Legend also has the Sultan entering the great church and uttering lines of a long forgotten Persian poem: "The Spider weaves his curtain in the palace of Caesars, the owl calls the watches in the towers of Afrasaib". Given Mehmed's own poetic output there's every chance he was moved to the lyrical by his achievement.
