Why not atrium?
Isn't atrium the accusative?
Yes, atrium is the accusative.
I think they are trying to say that 'we are walking, we are already inside the atrium' opposed to if the accusative was used 'we are walking into the atrium'.
This exercise really needs to be tidied to establish which case to use. Whether we are already in the atrium and walking or whether we walk into the atrium.
As is stands at present atrium and atrio could both be correct
If it was walking into then it would have used into.