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The Kerkoporta was a sally-port along the Theodosian Walls of Constantinople, near the bend created by the addition of the Blanchernae suburb to the original city. It was through this unattended gate that the first Turkic troops entered the city, raising their banner atop the tower and beginning the rout of the Christian defenders. Sections of the Theodosian walls of Constantinople as they appear today in suburban Istanbul The Walls of Constantinople surrounded the Roman and Byzantine city of Constantinople (today Istanbul in Turkey). ... Map of Constantinople. ...
The Byzantines also managed for a time to hold off the third attack by the Sultan's elite Janissaries, but a Genovese general in charge of a section of the defense, Giovanni Giustiniani, was grievously wounded during the attack, and his evacuation from the ramparts caused a panic in the ranks of the defenders.
Some historians suggest that the Kerkoporta gate in the Blachernae section had been left unlocked, and the Ottomans soon discovered this mistake (there was no question of bribery or deceit by the Ottomans; the gate had simply been overlooked, probably because rubble from a cannon attack had obscured or blocked the door).
Constantine XI himself led the last defense of the city, and throwing aside his purple regalia, dove headfirst into the rushing Ottomans, dying in the ensuing battle in the streets, like his soldiers.