The Long Walls were like something out of epic fantasy, with the added advantage that they were totally real. A few small parts of them still exist.
In this map, Athens is the pink blobby bit in the top right. Piraeus is the green blobby bit in the bottom left. Those two lines you see connecting them are the Long Walls. As you can see, they're long!
There was a wall built around central Athens, and Piraeus was a reinforced naval fortress. Athens, Piraeus and the Long Walls together formed one, huge, dumb-bell shaped fortification.
Why did they do this? Well, about two decades before my stories begin, a rather bright lad by the name of Themistocles realized that the safety of Athens required a massive fleet. I've previously talked about how huge the Athenian fleet was. At this time, they had about 200 triremes. They needed somewhere to dock and maintain all those boats, so Themistocles decided to turn a teensy fishing village called Piraeus into a massively fortified naval base, and in so doing he set the future of Greece for the next 2,500 years. To this day, Piraeus is their major port.
The only problem was the distance between Athens and Piraeus. An invading army -- Sparta would be a likely choice -- could stand in between to cut off the Athenians from their fleet.
So Themistocles decided to build the Long Walls. Even back then, they were sometimes called the Walls of Themistocles.
There's a paradox with the dating of these things. The historian Thucydides states that Themistocles caused the walls to be built, and he states that they were built in 458BC. The dating on this is technical. A Spartan army attacked to try to prevent the walls being completed, and it's possible to show this happened in 458, or roundabouts.
But Themistocles was ostracized in 470BC, 12 years before, and then condemned for treason. He never returned to Athens. How could he have caused walls to be built 12 years after he was gone? And why would the Athenians name a major defensive structure after a traitor? On the face of it, this is impossible!
Historians almost universally go with the 458 date, and don't have a good answer for the Themistocles question. But I wanted those walls to be there in 461BC, and besides, I think the date paradox needs to be solved.
It occurred to me that the walls might have been built twice. It might be that, after the Persian Wars, Themistocles caused a rapidly-constructed wooden wall to be built, and that subsequently it was replaced with a more permanent solution. Thucydides states point blank that the wall erected in 458 was of stone. A wood-to-stone conversion makes sense, especially since, after twenty years, they'd be starting to see maintenance issues.
Also, disregarding the recorded history, purely as a piece of military thinking, it's inconceivable to me that they would wait 20 years after the Persian Wars to build something so vital to their core naval strategy.
When I checked archaeological records (always check the archaeology) I discovered a report that suggested a small part of the remaining ruined wall structure seems to have been made of wood. That's not nearly enough to prove my theory by rigorous academic standards, but for a humble author of historical fiction, we're just fine.
Ta-da! Life is good.
So the walls Nico sees are specifically described as wooden, not stone.
If you'd been standing in the middle of the Long Walls back in 460BC, you would have seen huge amounts of traffic up and down, from the moment dawn broke. Fishermen landed their catch at Piraeus and carted it up to Athens, to sell in the agora. Workmen trundled their way back and forth. Navy men who lived in Athens walked down to their ships. Merchants with imported goods carted them off the cargo ships and up to the big city.
These days there's a bus route that runs where the Long Walls used to be, and I think the green line of the Athens metro is more or less on the same path too, but in Nico's day the only way up or down was to walk, or take a cart.