A sentry, watch, or guard.

A private soldier.

A unique string of characters recognised by a computer program for processing in a special way; a keyword.

“The tag is a sentinel that suspends web-page processing and displays the subsequent text literally.”

To watch over as a guard.

“He sentineled the north wall.”

To post as guard.

“He sentineled him on the north wall.”

To post a guard for.

“He sentineled the north wall with just one man.”