The Diary of Anne Frank is the famous story of a young Jewish girl and her family in hiding in the Secret Annex in the Netherlands during the Holocaust.  Because this story is so famous, and the Annex is now a very popular museum, keeping key aspects of the real place is very important to the telling of the story.  The Annex was a tiny, three story hidden apartment, above the offices.  The entrance to each room is from below, including the main space, with a staircase coming up from the trap.  Other elements remain the same, including Peter's bike on the wall by the stairs to the maps on the wall.  This creates the world where Anne is stuck, dreaming about a time when she could leave and explore the world.