IPA: /ˈɡɑːrdruːm/
KK: /ˈɡɑrdˌrum/
A room where guards work or where military prisoners are kept.
The soldiers waited in the guardroom before their shift began.
Guardroom → It is formed from "guard" (from Old French *garde*, meaning to watch or protect) and "room" (from Old English *rum*, meaning space or area). The word "guardroom" refers to a room designated for guards, where they watch over a specific area.
Think of a 'guard' who is there to 'watch' and 'protect' in a designated 'room' — that's why a guardroom is a space for guards.