Sentinel

1. One who watches or guards; specifically (Mil.), a soldier set to guard an army, camp, or other place, from surprise, to observe the approach of danger, and give notice of it; a sentry. (Mil.)

   The sentinels who paced the ramparts.

2. Watch; guard. [Obs.]
3. A marine crab (Podophthalmus vigil) native of the Indian Ocean, remarkable for the great length of its eyestalks; -- called also sentinel crab. (Zool.)

imp. & p. p. Sentineled; p. pr. & vb. n. Sentineling

1. To watch over like a sentinel. [R.]
2. To furnish with a sentinel; to place under the guard of a sentinel or sentinels.