Writers tend to describe the winter palace as containing three floors (note the difference in numbering between English/Russian from the standpoint of your character) while in reality it was six floors as mezzanines were built on each. Even with the renovations of 1920s-30s, some of the mezzanines have survived yet hidden. Also you have to look closely at the walls where hidden doors are part of the decoration. The Minister of the Court, for example Fredericks, could pop in and out of ceremonial occasions by the back corridor to his suite.
Some staircases (i.e. circular) were removed in 20s-30s but others remain yet hidden!
On state occasions the whole enfilade of rooms from the Neva side through the east rooms all the way around to the suite of Empress Marie A. were used including the lower Jordan gallery.
IM me if you have a particular question.