The German writer Norman Ohler lives on the top floor of a 19th-century apartment building on the south bank of the river Spree in Kreuzberg, Berlin. Visiting him there is a vertiginous experience. For one thing, he works – and likes to entertain visitors – in what he calls his "writing tower", a flimsy-seeming, glass-walled turret perched right on the very edge of the roof. (Look down, if you dare, and you will see his little boat moored far below.) For another, there is the fact that from this vantage point it is possible to discern two Berlins, one thrusting and breezy, the other spectral and grey. To our left, busy with traffic, is the Oberbaum Bridge, where there was once a cold war checkpoint, and beyond it the longest remaining section of the Berlin Wall, its doleful length rudely interrupted by the block of luxury flats that went up in 2013. As for the large building immediately opposite, these days it's the home of Universal Music. Not so very long ago, however, it was the GDR's egg storage facility.

Does all this press on Ohler as he sits at his desk, the light bouncing off the screen of his laptop? Is it ghostly sometimes? "Yes, it is strange," he says, smiling at my giddiness. But then he has long believed in a certain kind of time travel. "I remember the 90s. The wall had just come down, and I was experimenting with party drugs like ecstasy and LSD.