The old walls of a city are sometimes just picturesque reminders of the buried history of a place. But sometimes they mean more.
In the pre-dawn darkness, the first tram rumbles across the intersection between Torstrasse and Brunnenstrasse. The sound resonates in the empty halls of the U-Bahn station beneath, as though it is passing over the surface of a hollow drum.
There is one ever-present fact about Berlin, which may not be of quotidian interest to those on their way to work this morning, but which as much as anything else defines how it is seen.
Until thirty five years ago, it was not one city but two. Two cities which were not allowed to know each other. Separated it seemed as part of the collective punishment visited on Germany for the crimes of the Second World War.
One of the most unnerving experiences of my teenage years happened at a crossing point between West and East Berlin. I was part of a youth group which was putting on performances of the satirical musical, Oh What a Lovely War, in Hamburg and Berlin. On a day off, it was decided that we would make a trip to the communist side of the city.
All went well until we were being processed through the border by an East German policewoman. Perhaps it was the fact that I looked slightly hippyish, that I was wearing an army surplus jacket or that I was feeling quite tense in this unfamiliar situation. Whatever the reason, the stern-looking policewoman decided to take me aside for further questioning. As I sat waiting to be dealt with, I was terrified. I had no idea what I had done wrong or what might happen to me.
As things turned out, my rudimentary German was good enough to answer her questions in a way that did not get me into more trouble. But it left me with the impression of a state apparatus which might pick on someone just for being in some small way anomalous.
Back then, when the Wall still stood, when the Cold War was still being fought, it seemed almost miraculous to be able to see something of the other side. East Berlin was as exotic to us as the West must have been to Ostis. The tv tower which was the centre-piece of East Berlin was designed to be visible to and impress citizens on the other side of the Wall. It was, perhaps it did. But the cakes on sale in the tourist cafes were dry and tasteless and after only a couple of hours in the show-piece centre of East Berlin it began to feel like a soulless concrete desert. Such a stark contrast to the wild exuberance of West Berlin - the clubs, the squats, the drugs. As though a personality had been split in two. One half in the west, one in the east.
A couple of years later, as part of a programme organised by the German cultural agency, the Goethe Institute, I stayed in a country house on the edge of Berlin which was close to the Glienicke Bridge - the so-called bridge of spies. The location where prisoner swaps between east and west could take place under discrete and carefully managed conditions.
All this, the Wall, the Cold War, the division of Europe, all these things were immutable features of our world back then. Until they weren’t.
In the autumn of 1989, I was living in a small council house in Cardiff. I had just moved there to do a journalism course. The house in a poor area on the edge of town was cheap enough for me to afford with my grant and savings but by no means luxurious. In between me viewing the room in the summer and moving in a few weeks later, there was a rather serious fire in the kitchen. All of the ground floor was burnt out and uninhabitable. The acrid smell left by the fire reached up to the living room on the first floor, where we watched the news images showing people climbing on the Wall and hacking chunks off it, apparently intending to demolish it entirely.
By this time, I hadn’t been to Germany for more than six years. My connections and interest had faded. But it nevertheless resonated. Here was a place that I knew just a little bit, a nexus of world history and politics, changing in front of our eyes. As an aspiring journalist, I must have wanted to be in some way involved in the story, to use my knowledge and language skills, such as they were. But it was not possible. I had barely begun my course. I had a lot to learn.
Years passed. My life went in unexpected directions and I did not seriously think of returning to Germany until this year, when G and I wondered if it might be fun to recreate the Interrailing experiences of our youth. On a tour of the great cities of Europe, Berlin is an inevitable stopping-off point. Forty-one years had passed since my last visit. I had never known it as a unified city. My memories of a vanished, lost city would scrape against the new real one.
As it turned out, I found very few reference points - only the obvious ones like the Brandenburg Gate and the old tv tower. Everything else seemed to have changed.
The remnants of the Wall are sites for tourism and pilgrimage now. A thousand tiny pieces are on display in the shop windows along Friedrichstrasse where a replica of Checkpoint Charlie stands in place of the hugely complex transit zone which existed until 1989.
To own a fragment, to hold it in your hand, is to own a piece of history, a gritty chunk of freedom, the ultimate triumph of the West.
But how is the supply of product maintained? Do budding entrepreneurs sally forth at night to replenish their stocks, chipping pieces off the remaining old blocks of concrete, which stand defunct and isolated along the course of the old border.
In fact these ancient pre-loathed monoliths have been repurposed. To stand not just as symbols of the defeat of tyranny, but also as inspirations for those still fighting. For freedom of speech and thought, and above all perhaps for freedom of movement. Graffiti on one such segment of concrete outside the station on Potsdamer Platz memorialise the struggles of democracy activists in Belarus against the monster, Lukashenko.
It is striking that the Berlin Wall was built to keep people in, to stop them from escaping to the West, but the new barriers which have appeared on the borders of countries such as Hungary are intended to keep people out.
And further east, there is a new frontline. Not a cold war this time, but a raging furnace, consuming people and cities - capable, it seems, of destroying an entire nation. You thought the old conflict ended in 1989? Think again. Vladimir Putin is knocking at the gates of Europe. He wants to come in. And if we don’t let him, well, what then?
Political and media manipulation, black ops, all these failed to secure Ukraine for Putin. As he sees it, he was left with no alternative to military force. And there is no wall now between east and west, except that formed by the people of Ukraine, their will to resist and the will of the rest of Europe to help them.
Perhaps that is the lesson of the Wall. It created misery but also certainty. Like an old house badly partitioned to make two separate dwellings, nobody liked it, but they understood how it worked. Vladimir Putin is asking again what share of the common European home Russia is entitled to. The answer should leave as little doubt as the deal thrashed out by the great powers at the end of the second world war. But the hope of creating new certainties looks frighteningly distant.