Dazzling ... the ceiling of Sony Forum, Potsdamer Platz.Credit: Martin Moos/Lonely Planet
High on the wall is a cartoonish picture of a cat with its neck in a noose. "This guy doesn't like cats," Stu explains. "So he thinks of various ways to kill them. Some of them are really quite inventive but mostly he just hangs them."
Stu is an Irishman who came to Berlin to work as a financial analyst. He was made redundant when the hedge fund house of cards began to tumble and has since become engrossed in Berlin's street-art scene. He now earns a living taking visitors around the "alternative" Berlin the squats, the artist communes, the independent galleries and, of course, walls covered in graffiti of varying quality.
"I used to go to parties," he says. "And people would look at me strangely when I told them what I did. No one works in an office here they're all poets, artists or DJs."
So he became sucked in. He can now explain the differences between tags, throw-ups, stencils and bombing; he can identify the individual artists; he can differentiate quality of paintwork; he can tut loudly when he sees a breach of unwritten protocol.
He reserves his biggest condemnation for what has happened to Berlin's most famous street-art showground. The East Side Gallery is one of the few remaining stretches of the Berlin Wall. It lines the banks of the River Spree, dividing Friedrichshain in the former East Berlin from Kreuzberg on the western side.
When the Wall was torn down the 20th anniversary comes around in November this year this small stretch was left for local artists. Each was given a small section and told to go for their lives. The result was a series of murals, largely with themes of peace. Unfortunately, since then, they have been mostly covered over by mindless tagging and poor-quality splodgery from people wanting to make a name for themselves. "It's the bit that everyone wants to see," says Stu.
"But it's something of an anticlimax. Unfortunately, too many idiots want to write 'I was here' on the Berlin Wall."
In the two decades since the Berlin Wall came down, the city has really moved on. The museum at Checkpoint Charlie, detailing the escape attempts and history of the Wall, is still fascinating once you fight your way around the marauding school groups but it seems old hat now. Berlin isn't about the Wall any more.
This is partly because many of the city's blockbuster attractions are within the parts of East Berlin which were, to all intents and purposes, out of bounds before 1989.
Museum Island is a case in point. It currently boasts four of the world's greatest museums which have had extensive restoration makeovers since German reunification and another one is on the way. It would be easy enough to spend a day there, flitting from the old masters in the Alte Nationalgalerie to the art and architecture of ancient civilisations in the Pergamon.
Nearby is Hackesche Hofe, a network of courtyards that has also had the money thrown at it. It's now the prime shopping, dining and mooching complex that it was in the past.
It's one of Berlin's many, many nightlife hotspots and the vast majority of these are in the eastern half of the city. The truly hip scene shifts roughly every 30 seconds but it's hard to go wrong with Scheunenviertel, which as a very rough guide covers the area between Hackesche Hofe and Oranienburger Tor.
It's an area crammed with galleries, cafes and bars and plenty of street art, as Stu points out while we're walking through. He spots one with a dashed box around it and a little gift tag emblem saying "Cut & Go". Apparently, this is a badge of honour there's one anonymous chap who puts the Cut & Go boxes around what he deems to be the best pieces of work.
The signs advertising the prices of happy hour cocktails are ubiquitous and thoroughly dangerous. Scheunenviertel is one of those places where once you start, it's highly unlikely you'll be back in the hotel before the early hours.
The coolest bar of the lot is Zapata. It has a grungy, arty vibe created partly by the clientele and the decor but largely by the building it is in.
Kunsthaus Tacheles looks incongruous in its surroundings. It's an extremely dirty and dishevelled former department store and would be a complete eyesore were it not for a unique Berlin phenomenon.
When the Wall was up, the areas in the immediate vicinity weren't exactly prime real estate. Along much of the Wall's path, there was urban decay; buildings nearby were left to ruin. This is partly because the East German authorities wanted to create a no-go zone around the Wall and the best examples of this can be found on Bernauer Strasse slightly further north. The Wall Memorial is here and it can be viewed from a high platform on the other side of the road.
Essentially it's a remaining part of the wall, plus a strip of no man's land behind it. Walking up the street it's easy to see the divide. On the west side, buildings come to the edge, on the east side, there are scrubby patches of rubble and the occasional dilapidated house.
When the Wall came down, the first people to move into the territory surrounding it were the squatters and the artists. The buildings became illegal homes, studios, galleries and bars. And of course, they were the coolest, edgiest hang-outs. The most visible example of this is by the River Spree near the East Side Gallery. On the Kreuzberg side, a series of squats still line the riverbank, while on the Friedrichshain side are the warehouses that morphed into huge clubs in the 1990s.
Of course, with the Wall gone, the areas it once cut through are now highly sought-after. One by one, the artist squats have been shut down and multimillion-euro loft-apartment and office-block developments have taken their place.
The most enormous makeover has been performed at Potsdamer Platz. Always a busy transport hub, it was bisected by the Wall and became a giant wasteland as a result.
Now it is home to shopping malls, IMAX theatres, a gargantuan entertainment complex and gleaming glass skyscrapers. It's architecturally dazzling but, while mainstream Berlin has happily adopted it, Potsdamer Platz isn't exactly a byword for soul.
Kunsthaus Tacheles, therefore, is the last great outpost of Berlin's immediate post-Wall zeitgeist. In the sense that people no longer live there, it is technically no longer a squat but the artists and craftspeople who have taken it over don't pay any rent. It is covered in graffiti and could do with a good tidy-up.
Zapata is merely a part of what Tacheles is about. Walk through the archway to the land at the back and there's a showroom made from shipping containers and plastic sheets. Inside there are some astonishing sculptures carved out of scrap metal, the biggest one being a giant cockerel.
Inside the building and up the litter-strewn stairs are a series of exhibition spaces, all belonging to an individual artist.
Some of the work on display is truly superb.
The collages with a political edge made by Tim Roeloffs are refreshingly different, while Belarusian artist Alexander Rodin's incredibly detailed and somewhat sinister paintings have a genuine wow-factor.
Unfortunately, Kunsthaus Tacheles is on borrowed time. The artists have come up with petitions, action plans and marketing schemes to save it but in all likelihood, it will not be there two years from now. Another big money development is on the cards and a chapter in Berlin's first 20years after the Wall will come to an end.
The art scene will live on, of course. Creativity is in Berlin's blood and the hotspots will simply move to another area probably further east. But for now, despite enormous changes taking place since 1989, great things still grow in the shadow of the Wall.
There are no one-stop flights to Berlin from Sydney but Lufthansa, British Airways and Thai Airways all offer reasonably scheduled two-stop flights for under $2400.
Ostel is part of a growing movement called Ostalgie - essentially a celebration of culture from the days of East Germany. It is in a renovated tower block near the East Side Gallery and is furnished in GDR-style pastiche. An ensuite double costs €61 ($108) a night. See ostel.eu.
Kunsthaus Tacheles (tacheles.de) is on Oranienburger Strasse. It can be visited as part of the Alternative Berlin walking tour with Sandeman's New Berlin Tours (newberlintours.com). It lasts about 3½ hours, costs €12 and includes street art and the East Side Gallery.
Berlin.de has details on Potsdamer Platz, the memorial on Bernauer Strasse, the Museum at Checkpoint Charlie and Berlin's other highlights. For information on 20th-anniversary events, see mauerfall09.de.
Sign up for the Traveller Deals newsletter
Get exclusive travel deals delivered straight to your inbox. Sign up now.