guards — to keep safe from harm or danger; protect; watch over: to guard the ruler.
sentries — a soldier stationed at a place to stand guard and prevent the passage of unauthorized persons, watch for fires, etc., especially a sentinel stationed at a pass, gate, opening in a defense work, or the like.
spies — a person employed by a government to obtain secret information or intelligence about another, usually hostile, country, especially with reference to military or naval affairs.
watches — to be alertly on the lookout, look attentively, or observe, as to see what comes, is done, or happens: to watch while an experiment is performed.