sentry
n.a soldier or guard whose job is to stand at a gate or entrance and watch for danger.
n.a soldier stationed to keep guard or to control access to a specific location.
“The sentry stands at the main gate.”
“The sentry challenged the approaching vehicle and asked the driver for identification before opening the gate.”
“Throughout the long winter night, the lone sentry paced the perimeter of the outpost, scanning the dark horizon for any sign of movement that might signal an approaching threat.”
Often used with the verb 'to stand' ('to stand sentry').
“The threat from their own side was just as real: French and British sentries, awash in rumors of infiltrators and unnerved by automobiles, often panicked at checkpoints.”
From earlier sentrie, sentery, of uncertain origin. Perhaps an alteration of sentinel or sanctuary; or perhaps from Old French senteret (“a path”), diminutive of sentier, from Medieval Latin semitarius (“a path”).
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.