What is another word for "sentinel"?

Roget's Thesaurus (1911) lists these synonyms for "sentinel": and, ayah, behold, beholder, bodyguard, bonne, bystander, castellan, coast, concierge, costa, custodian, custos, duenna, escort, eyewitness, game, gaoler, gawk, governess, governor, guard, guardian, hayward, jailer, keeper, look, nanny, nurse, observer, passer, present, protector, ranger, rubber, see, sentry, spectator, spy, teacher.