Last week the world celebrated the 20th anniversary of the fall of the Berlin wall, when armed guards stood aside as a trickle of people became a flood, pouring across the border from the East. At that moment East and West Berlin ceased to exist, a nation was reunited with itself and the Cold War was over.

Since 1961, West Berlin had been a psychological construction as much as a tangible, physical city, existing in the mind as much as in reality. If ever a city had acted as a mirror to the unconscious, this was it. "People never stop partying in West Berlin!" people would say. "You must go there - you've got to experience it! The atmosphere is incredible! It's so hedonistic!" But what exactly created this atmosphere?

One rainy night in November 1987 I found myself in Amsterdam station. The day had not gone well. Earlier two North African men had flicked a knife under my overcoat in the Damrak while I paused in front of a shop window. Money had been demanded, I had resisted, but the knife rose to my jugular. I had surrendered.

Later, with my one remaining credit card (hidden in my luggage), a police report and a European rail pass, I stared at the indicator board in the station. I needed to get out of here! Anywhere! The letters on the board clacked and crackled into life - spinning and clicking, finally settling on the destination: "West Berlin". An island city! A walled city!

In 1987 West Berlin really was an island - a capitalist enclave hundreds of miles inside communist East Germany. The only way in was by plane, or through a few heavily-guarded rail or road "corridors". For the citizens of the East, there was no way out. Berlin was schizophrenic - a wall separated East and West, there were bombed-out church spires, disused tunnels, abandoned government buildings. It was a city of ghosts, a fantasy city of the imagination.

As the train rolled through the damp Dutch night I settled into my seat - there was no budget for a bed - enjoying the empty compartment and the silent night. Even the fact of nearly being murdered seemed less shocking now.

The silence was shattered as five British soldiers threw the door open. These Cold War warriors opened beer cans, shouted questions "Where you from mate?! What the **** are you doing here" One unscrewed the light bulb; another prised a picture off the compartment wall as a souvenir. They swore about the Germans, they sang rude songs but, at Osnabruck, they vanished from the train as quickly as they had appeared, laughing into the night...

Next morning, after we pulled into Berlin's "Zoo" station, I took a walk in the Tiergarten, found lodgings in Spandau - famous for its prison that had housed just one inmate, the Nazi Rudolf Hess, for over 20 years. Now, though, the prison lay empty - Hess had died three months previously. A prison with one inmate, an empty jail - this could only make sense in the battered psyche of West Berlin.

The Wall, Checkpoint Charlie, the watchtowers and the grim-faced guards: what had once been the centre of the old city was now a no-man's land of barbed wire, search lights and guard dogs. The area was mined: escapees were shot on sight.

The cracked psychology was emphasised by the subway. The Wall did not follow a straight line, so an ordinary subway ride in West Berlin could pass through, or rather under, the East, where stations had been sealed off to avoid people escaping. As we passed through these "ghost" stations, the train slowed, revealing a spectacle of dimly lit platforms, 1961 advertising hoardings and bored communist guards. The West Berliners paid no attention and continued on their capitalist carousel: humans will get used to anything.

In the middle of this ghost-train subway ride was Friedrichstrasse, where the train DID stop. Here were the border controls, here was Checkpoint Charlie, where I exchanged 25 Deutschmarks (compulsory), got a stamp in my passport, and walked into the communist zone. I found a world of drab shops, unspoilt architecture, almost no cars and few people on the streets.

There was little choice in the restaurants, but what there was cost almost nothing. There was no mad consumerism, no expensive department stores, flashy sports cars or designer clothing. People didn't look particularly happy, but they also seemed calmer, less stressed than their western counterparts. The atmosphere was tranquil, peaceful almost.

That afternoon I had spent only 10 of my 25 Marks (there was nothing to buy) but taking money out of East Berlin was forbidden. There were no tramps or beggars on the streets, so I left the rest under a lamp post, and walked back through Checkpoint Charlie.

Back in 1987 the two parts of this city really were like a fractured psyche - a yin missing its yang, a conscious without an unconscious. Each part existed only in relation to its opposite: neither was complete without the other.

On reflection, Cold War Berlin was an extreme example, but really ALL cities can be thought of in this way. The way a city is constructed, with its underground tunnels, passageways, box-like offices and skyscraper apartments, the way most of it is hidden from view, unknowable, compartmentalised. We can only ever know a tiny part of its reality.

And come to think of it, isn't this just a replica of our own brains?