Looking up from the street, this concrete, angular building looks no different from the many Brutalist tower blocks dotted around Belgrade.
As you go inside, a prison-style gate with vertical metal bars guards the white front door of the first flat on the left.
This was the home of Novak Djokovic's grandfather Vladimir.
Here, the world's leading male tennis player sheltered as a small child while Nato bombed the Serbian capital between March and June 1999.
When the head-pounding drone of air-raid sirens rang out, families spanning several generations, along with neighbours and friends from nearby blocks, all filed down the stairs, through several steel doors and into the basement.