noun
antechamber
AN-tee-chaym-ber/ˈæntitʃeɪmbər/
A small room leading into a larger or main room.
The small antechamber opens through grand doors into the throne hall.

Etymology
Here “before” describes position rather than time: an antechamber is a room in front of the principal room, providing access to it and often serving as a waiting room.
In context
the small room that you pass through from the grand gallery before entering the granite line king's chamber it's called the antechamber
More examples
- A guard admits them from the antechamber.
- The palace contains several richly decorated antechambers.
- She crosses the small antechamber before entering the hall.
Forms
antechamber · antechambers
Similar words
- vestibule
- Usually a small entrance space immediately inside a building.
- waiting room
- Specifies a use rather than a position before another room.