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sentinel (noun) sentry or guard.
A sentinel is a sentry or guard. A sentry does the duty of guarding a place and prevents the entry of enemies into the place.
- They posted too many sentinels around the castle for any enemy to enter it. They were guarding the castle with all alertness.
- The sentinels around the place were not as watchful as sentinels should be. It was peacetime, so most of them thought there was no need to guard the place so carefully.