sentry

sentry
/ˈsentrɪ; `sɛntrɪ/ n
soldier posted outside a building, etc in order to watch or guard it 岗哨; 哨兵
    People approaching the gate were challenged by the sentry. 向大门走去的人受到了哨兵的查问.
    [attrib 作定语]
sentry duty 哨兵的值勤.

牛津英汉双解词典