It was a week before Christmas outside the bomb-damaged Kaiser Wilhelm church and the heavy boots of the riot police were caked with snow. Behind the line of olive green uniforms, helmets and shields, families were munching currywurst and sipping gluhwein to the strains of O Tannenbaum, a German Christmas song with the same melody as The Red Flag.
This was 1985 and I had arrived in Berlin just five hours earlier after an overnight train journey through East Germany, during which grumpy border guards patrolled the corridors as we slept.
Although I had visited the city a few years earlier, I think of this image of the Christmas market around the ruined church as my first memory of Berlin.
The church, which stands at the top of the Kurfurstendamm, West Berlin's main shopping street, was deliberately left in its damaged state as a reminder of the misery of war. The riot police were on duty to protect visiting world leaders from the city's hard core of violent anarchists who had vowed to wreck an economic summit being held nearby.
Cold War Berlin was such an extraordinary place that, looking back, it is difficult to imagine how it survived for so long. A walled island in the middle of the communist east, West Berlin was a bizarre and contradictory showcase for western democracy where the most important freedom for many citizens was the right to stay out all night long.
I got to know dozens of these committed idlers during my first years in the city and learnt to marvel at the ingenuity of their schemes to stay on at university for yet another year while they gained a cult status working two nights a week in a fashionable bar.
If you crossed Checkpoint Charlie into East Berlin, as I did every few weeks, it seemed as if the clock was frozen at a moment not too long ago but that nobody quite remembered. The German Democratic Republic was, as the novelist Stefan Heym observed, not very democratic and not really a republic but very German indeed.
Isolation from western advertising and the fact that few easterners spoke English meant that the East seemed to be culturally more German - or more Prussian, perhaps - than the west. But as I got to know a few young people, went to their bars and discos and back to their homes, I was reminded of the Ireland I had left behind.
Even after Gorbachev came to power in the Soviet Union there was little sign that change was on the way in Erich Honecker's East Berlin. Honecker was determined to resist the perestroika reforms and the words every East German child chanted at school - "to learn from Russia is to learn to win" - became a subversive slogan.
When the end came, it came so fast and so dramatically that few Germans understood quite what was going on.
For a few months, it seemed that two systemsand two states might survive and even Helmut Kohl hesitated before he launched his campaign to unify Germany as quickly as possible.
Ruthless, selfish and intellectually uninspiring, Kohl is an unlikely hero, but by grasping the moment of historical opportunity he ensured that, regardless of the outcome of the current investigations into his fundraising irregularities, his reputation as one of the great German political leaders is secure. Berlin was where unification had its most immediate impact, so that westerners and easterners found themselves meeting every day. The more they got to know each other, the less they liked one another and both sides soon started to murmur that they wanted the Wall put back up again.
Westerners characterised their eastern neighbours as lazy, ungrateful and greedy while East Berliners complained of western arrogance and a lack of understanding of the difficulty of adopting an entirely new system. On balance, my sympathy is with the easterners, who had to adjust to new ways of doing everything from insuring their houses to claiming state benefits.
Some easterners, however, expressed their bitterness by turning on foreigners, and ethnic minorities throughout eastern Germany live in almost constant fear.
Chancellor Gerhard Schroder is showing more political leadership in the fight against racist violence than his predecessor did. But the conservative opposition appears determined to use immigration and its threat to what they call Germany's "defining culture" in the next election campaign.
The need to attract more workers from abroad may force German society to confront the reality that it is a multicultural society and that it benefits from ethnic diversity. But the Chancellor will need a strong political nerve to face down populist demands for more safeguards for German culture that can only have the effect of alienating ethnic minorities further.
Leaving Berlin after such a long time, I am struck by how little difference the changes of the past few years have made to how I think of the city. The massive construction programme of the 1990s has transformed the centre of the city, and Potsdamer Platz, long a wilderness in the no man's land between east and west, is now a huge, spanking new complex of shops, offices and cinemas.
But as I cycle through the Tiergarten, spend an evening at the Paris Bar or stop off in a kneipe for a late night drink, Berlin is what it has always been to me - a city where you can be yourself, no matter how unusual or outrageous that should be. It is a city that has taken to heart Frederick the Great's promise of freedom and diversity in Prussia: Jeder nach sein Facon (Each after his own fashion).