A guardhouse is a building for soldiers who are guarding the entrance to a military camp — or another place that needs a watch at the gate. The soldiers on duty sit or stand there; it is their post by the entrance, where they check who comes in. You will see one at a barracks, a fort, or sometimes a prison. It is a compound: a house for the guard, usually small. Though actually, in some armies the same word has also been used for a building where soldiers are locked up as punishment. That is a different job. The meaning here is the building for the people doing the guarding, not a jail. It is not a house that is being guarded; it is the house the guard uses.

衛兵室