Constantinople fell because someone left a gate wide open.
The fortifications of Constantinople were built to be impregnable. Which was just as well, because, throughout history, there were plenty who tried to take the capital of the Eastern Roman Empire.
The forces of the Kutrigurs, Avars, Slavs, Sassanid Persians, Umayyad Caliphate, Krum of Bulgaria, Rus’ and The Fourth Crusades in 1203 all tried and failed to take Constantinople. For the fortress’ walls were impenetrable. But there was one weakness in the colossal fortress: the gates.
On the morning of 29 May 1453, the small gate to the city known as the Kerkoporta was left wide open. This was a gift to the forces of the Ottoman Empire who waited outside. Fifty or more were able to enter undetected. They raced through the city and planted their banner to signal to their forces that they were inside Constantinople.
Shots were fired between the Turkish Invaders and the Greek defenders of the peribolos. Panic spread and the city the Ottomans took the city.
But Scholars continue debating if Constantinople really captured because of such a careless act. Certainly, the siege occurred. But because most of the fortress is now gone, no one knows for certain the location of the gate that may have brought about the fall of the Roman empire.