The city-fort, protected by the most imposing walls known to history to that time, and in the center of a strategic waterway, was still Constantinople, at least for a little while longer, when Sultan Mehmed II lay siege to it. Mehmed undoubtedly knew the city had only fallen once before, despite countless attempts, but the burgeoning Ottoman Empire could not just leave the last redoubt of the Roman Empire alone. The force he assembled to take the city was vastly numerically superior, but the Christians had the advantage of defending a fortified spot.

On this day, April 6, in 1453, Sultan Mehmed II began the final siege of Constantinople. An artillery bombardment weakened portions of the great walls before the first suicide-squad infantry rushed in, followed by more focused attacks on the damaged sections breached the walls. Still, all were repelled. The fall of Constantinople came from a gate neglectfully left open. Mehmed’s soldiers rushed in after discovering the gate and the rest was history.

The fall of Constantinople marked the end of the 1,500-year Roman Empire and the rise of the Ottomans in their stead. With Constantinople, the Christians lost the cathedral of Hagia Sophia, one of the oldest structures of Christendom (built by the Byzantium Greeks in the 300s A.D.), as well as numerous religious treasures. The Christian residents of Constantinople who made it out alive embarked for Europe, which some historians suggest helped spark the Renaissance.