The Collaborative International Dictionary of English v.0.48:

Sentinel \Sen"ti*nel\, v. t. [imp. & p. p. Sentineledor
   Sentinelled; p. pr. & vb. n. Sentineling or
   Sentinelling.]
   1. To watch over like a sentinel. "To sentinel enchanted
      land." [R.] --Sir W. Scott.
      [1913 Webster]

   2. To furnish with a sentinel; to place under the guard of a
      sentinel or sentinels.
      [1913 Webster]