guard
person who is present in (or near) buildings, territory, and other property to provide security
sentinel
guard, particularly on duty at the entrance to a military base
crossing keeper
person who controls the passage of a train by a level crossing
lookout
person on a ship in charge of the observation of the sea for hazards
warden
custodian, defender, or guardian
military guard
type of military unit and position
night watchman
law enforcement occupation