They surround the modern city of Istanbul, Constantinople as the Romans called it.

And for a thousand years, the fate of Europe depended on them.

Constantinople was designed to be the center of the world.

When the frontiers of the Roman Empire began to crumble in the 4th Century,

the capital was moved to the cultured, wealthy, and still stable East.

There, at the crossroads between Europe and Asia, the hub of the major trade routes of the ancient world,

the Emperor Constantine built his city.

This was the city of libraries and universities,

20 times the size of London or Paris at the time.

It contained the priceless knowledge of the classical world which was fading in the West.

To protect this masterpiece from its many enemies,

Constantine's successors built the finest defensive fortifications ever made.

The first line of protection was a moat 60 feet wide and 22 feet deep,

stretching all four miles from coast to coast.

Pipes from inside the city could fill it at the first sight of the enemy,

and a short wall protected archers who could fire at the soaked soldiers trying to swim across.

Those who were lucky enough to clear the moat had to contend with an unceasing barrage from the 27 foot outer wall above.

Arrows, spears, or far worse, Greek fire -- an ancient form of napalm that would ignite on contact

and couldn't be extinguished by water -- would rain down on them.

Squads of Roman defenders would carry portable flame throwers,

spraying anyone trying to climb out of the moat.

The terrified victims would leap back, only to find that they still burned underwater.

At times, the Romans would also mount siphons onto the ramparts,

and launch clay pots full of Greek fire from catapults at an invading army.

The front lines would turn into an inferno,

making it appear as if the earth itself was on fire.

If, by some miracle, the outer wall was compromised,

attackers would be faced with the final defense: the great inner wall.

These walls were wide enough to have four men ride side by side,

allowing troops to be rushed wherever they were needed.

Attilla the Hun, destroyer of civilizations, who named himself the Scourge of God,

took one look at them and turned around.

The Avars battled the walls uselessly til their catapults ran out of rocks.

The Turks tried to tunnel under them, but found the foundations too solid.

The Arabs tried to starve the city into submission,

but ran out of food themselves and had to resort to cannibalism.

It took the guns of the modern world to finally bring them down.

In 1453, the Turks brought their super weapon:

a monster cannon that could fire a 15 hundred pound stone ball over a mile.

Together with more than a hundred smaller guns,

they kept up a steady bombardment day and night.

A section of the old walls collapsed, but even in their death throes they proved formidable.

The rubble absorbed the shock of the cannonballs better than the solid wall.

It took a month and a half of continuous blasting to finally open a breach.

The last Roman Emperor, Constantine the 11th, drew his sword

and jumped into the gap to stop the onrushing horde,

disappearing into legend.

The city was taken, and the Roman Empire finally disappeared.

But those broken walls had one last gift.

As the survivors fled the doomed city, they brought with them their precious books and their ancient traditions.

They traveled west to Italy, reintroduced the Greek language and learning to western Europe, and ignited the Renaissance.

Thanks to Constantinople's walls, that pile of brick and marble that guarded them for so long,

we still have our classical past.