For more than a thousand years, Constantinople had stood as one of the greatest cities in the world.

Founded as the eastern capital of the Roman Empire, it became the heart of the Byzantine Empire and a center of trade, religion, politics, and culture. Its location was extraordinary. The city sat between Europe and Asia, surrounded by water on three sides, with the Bosporus to the east, the Sea of Marmara to the south, and the Golden Horn to the north.

And protecting the city from the west were the massive Theodosian Walls.

Built in the fifth century, these walls had defended Constantinople for centuries. They stretched for miles and were made up of multiple layers of fortification—an outer wall, a moat, and a towering inner wall reinforced with dozens of defensive towers.

Invading armies had tried repeatedly to break through them.

Most had failed.

By the middle of the 15th century, however, the once enormous Byzantine Empire had been reduced to little more than Constantinople and a handful of surrounding territories. The Ottoman Empire, meanwhile, had expanded across much of the region.

In 1451, a young Ottoman sultan named Mehmed II came to the throne.

He was only nineteen years old.

And he wanted Constantinople.

Capturing the city would give the Ottomans control over one of the most strategically important places in the world and remove the last major Byzantine stronghold surrounded by Ottoman territory.

Mehmed began preparing almost immediately.

One of his most important projects was the construction of the fortress now known as Rumeli Hisarı on the European side of the Bosporus. Built directly across from an existing Ottoman fortress on the Asian shore, it allowed Mehmed to control shipping through the strait and make it far more difficult for supplies and reinforcements to reach Constantinople by sea.

He also assembled a massive army.

Among his weapons were enormous cannons capable of firing stone balls against fortifications that had been designed centuries before the age of heavy gunpowder artillery.

In April 1453, the Ottoman army arrived outside Constantinople.

Inside the city, Byzantine Emperor Constantine XI prepared for what everyone knew could be the final siege.

The defenders were dramatically outnumbered, but they had one enormous advantage.

The walls.

For weeks, Ottoman cannons pounded the fortifications while Byzantine soldiers repaired the damage as quickly as possible. Assault after assault was pushed back.

But Constantinople had another important line of defense.

The Golden Horn.

This long natural harbor wrapped around the northern side of the historic city. To prevent enemy ships from entering it, the Byzantines stretched a huge iron chain across its entrance between Constantinople and the settlement of Galata on the opposite shore.

The chain could be raised to block ships from sailing into the harbor.

It worked.

Ottoman ships could not get past it.

That created a serious problem for Mehmed. As long as the Byzantines controlled the Golden Horn, the northern walls of Constantinople were relatively safe, and ships friendly to the defenders could still use portions of the harbor.

So Mehmed came up with an extraordinary solution.

If his ships could not sail through the entrance to the Golden Horn, he would simply take them around it.

Over land.

During the siege, Ottoman engineers prepared a route across the hills behind Galata. Wooden tracks were laid down and coated with grease to reduce friction.

Then, under cover of darkness, Ottoman ships were pulled out of the Bosporus and hauled across the land.

Men and animals dragged the vessels over the hills.

Ships that had been floating in the Bosporus were suddenly moving across dry ground.

By the following morning, dozens of Ottoman vessels had been lowered into the Golden Horn—behind the great chain that had been designed to keep them out.

The Byzantines were stunned.

The defensive barrier had not been broken.

It had simply been bypassed.

The arrival of Ottoman ships inside the Golden Horn changed the siege. Constantinople’s defenders were now forced to protect additional sections of the city walls, stretching their already limited forces even thinner.

Still, they continued to resist.

For nearly two months, the siege dragged on.

Cannons continued hammering the walls. Ottoman soldiers attempted to tunnel beneath the fortifications. Byzantine defenders repaired breaches, fought off assaults, and tried desperately to keep the city from falling.

But the pressure was becoming impossible to sustain.

In the early hours of May 29, 1453, Mehmed launched his final assault.

Wave after wave of Ottoman troops attacked the walls.

The defenders fought desperately, but eventually Ottoman forces managed to break through.

Emperor Constantine XI is believed to have died fighting during the final defense of the city.

By morning, Constantinople had fallen.

The Byzantine Empire, whose history stretched back more than a thousand years, had come to an end.

Mehmed II entered the city as a conqueror.

He was only twenty-one years old.

The capture of Constantinople earned him the name Mehmed the Conqueror and transformed the city into the new center of the Ottoman Empire.

One of his most symbolic acts took place at Hagia Sophia.

For nearly nine hundred years, the enormous domed building had stood as one of the greatest churches of the Christian world and the spiritual heart of the Byzantine Empire.

After the conquest, it was converted into a mosque.

Over the centuries that followed, Ottoman rulers reshaped Constantinople. Mosques, palaces, markets, fountains, schools, baths, and neighborhoods were constructed throughout the city.

Topkapı Palace became the center of Ottoman government.

The Grand Bazaar grew into one of the great commercial centers of the empire.

Mosques eventually filled the skyline alongside the surviving Byzantine structures.

But the older city never completely disappeared.

The walls still stood.

Hagia Sophia remained.

Roman cisterns survived beneath the streets.

Byzantine churches, columns, ruins, and foundations became part of the expanding Ottoman capital.

That mixture is one of the reasons Istanbul feels so layered today.

The city travelers walk through is not simply Byzantine or Ottoman.

It is both.

And the events of 1453 form one of the most important dividing lines between those two histories.

The fall of Constantinople also had consequences far beyond the city itself. Ottoman power expanded further into southeastern Europe and the eastern Mediterranean, and Constantinople became the political center of an empire that would endure for centuries.

The city’s position between Europe and Asia continued to make it one of the world’s most important centers of trade and culture.

Centuries later, Constantinople would eventually become known internationally as Istanbul.

But perhaps the most unforgettable detail of the entire conquest remains the moment Mehmed looked at an enormous chain blocking his fleet from the Golden Horn and decided that the sea was not the only way to move a ship.

Today, visitors to Istanbul can still see surviving sections of the Theodosian Walls. They can stand beside the Golden Horn, explore Rumeli Hisarı, walk through Hagia Sophia, and look across a city whose skyline still reflects both the Byzantine and Ottoman worlds.

It is difficult to imagine that more than five centuries ago, Ottoman ships were being hauled across the hills surrounding that same harbor.

But they were.

The chain held.

The ships went around it.

And on May 29, 1453, one of the longest-lasting empires in history came to an end while another entered a new era.

When the sea was blocked, Mehmed II moved his ships over the land—and changed the history of the city forever.