It was a moment I will never forget.

We had just rounded the curve in London's Trinity Square Gardens, and there it was, a massive

stretch of Roman Wall, in the middle of a modern city.

Every single brick seemed to have been highlighted by centuries of erosion and exposure, but it

still stands tall, bringing history into the present.

Of course, the more knowledgeable it would be quick to point out that only the first four

meters of the wall are original.

The top 60% or so, while still ancient, only date back to medieval times.

But beyond the numbers is a larger message.

Long ago, this wall contained a city.

And now, well, that city has swallowed it whole.

And I think that's the thing that jumped out at me the most when I first saw it.

Not its age, or its ancient design, but its place in our world today.

Walls are meant to create boundaries to hedge people in and even to defend against outside

forces, so it felt odd to see this fragment of a wall surrounded by gleaming modern office

buildings, instead of wrapping it all up in a protective embrace.

See, that's because the walls of history often seem larger than life.

The great wall of China, for instance, is visible from space.

The walls of Troy are some of the oldest in the world, while the ruins of Hadrian's walls

still cut across the countryside between England and Scotland.

And then of course, there's the western wall of the temple in the old city of Jerusalem,

and the Berlin wall of the Cold War era.

...