The Collaborative International Dictionary of English v.0.48:

Warder \Ward"er\, n.
   1. One who wards or keeps; a keeper; a guard. "The warders of
      the gate." --Dryden.
      [1913 Webster]

   2. A truncheon or staff carried by a king or a commander in
      chief, and used in signaling his will.
      [1913 Webster]

            When, lo! the king suddenly changed his mind,
            Casts down his warder to arrest them there.
                                                  --Daniel.
      [1913 Webster]

            Wafting his warder thrice about his head,
            He cast it up with his auspicious hand,
            Which was the signal, through the English spread,
            This they should charge.              --Drayton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

warder
    n 1: a person who works in a prison and is in charge of
         prisoners