A sentry, watch, or guard.
"The old oak tree stood as a silent sentinel over the quiet village square."
In plain English: A sentinel is someone who stands guard to watch for danger and warn others if something bad happens.
"The large statue stood like a sentinel guarding the entrance to the museum."
To watch over as a guard.
"The security team assigned a sentinel to watch over the main entrance throughout the night."
A ghost town in California.
"The rusted skeleton of Sentinel stands as a lonely reminder of the mining boom days along the California coast."
The word sentinel entered English in the 1570s from Middle French and Old Italian, where it originally referred to someone who feels or perceives things through their senses. Its ultimate root is Latin sentiō, meaning "to feel" or "perceive by the senses."