The Fall of Constantinople occurred in 1453.
The decisive siege began on April 6 and ended on May 29, when Ottoman forces under Sultan Mehmed II captured Constantinople, capital of the Byzantine Empire. The city’s last Byzantine ruler, Emperor Constantine XI Palaiologos, died during the final defense.
Constantinople’s massive Theodosian Walls had protected the city for centuries, but Ottoman gunpowder artillery helped breach them. Mehmed then made the city his capital, replacing Adrianople. The conquest effectively ended the Byzantine Empire and is often used by historians as a marker for the end of the medieval period.
A common mix-up is confusing this event with the Fourth Crusade’s capture of Constantinople in 1204. That earlier occupation was temporary; the Ottoman conquest in 1453 was permanent and transformed the city’s political future.