Where is the spiral staircase in Harry Potter and the Order of the Phoenix?

Answer 1

It's on the bottom floor. Go up ONE set of stair and you'll se a portrait of a red bearded man. Talk to him.

Answer 2

St. Paul's Cathedral London. This is the Geometric Staircase in the South Bell Tower which goes to the dome at St. Paul's Cathedral in London.

Answer 3

In the video game, go to the opposite wall from where Hannah is, and climb up the stairs. It's the first portrait you see.