The Collaborative International Dictionary of English v.0.48:

Watchtower \Watch"tow`er\, n.
   A tower in which a sentinel is placed to watch for enemies,
   the approach of danger, or the like.
   [1913 Webster]

WordNet (r) 3.0 (2006):

watchtower
    n 1: an observation tower for a lookout to watch over prisoners
         or watch for fires or enemies

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "watchtower":
   Texas tower, beacon, belvedere, bench mark, bleachers, bookmark,
   bridge, cairn, catstone, conning tower, gallery, gazebo,
   grandstand, landmark, lighthouse, lightship, lookout, loophole,
   mark, marker, menhir, milepost, milestone, monument,
   observation post, observatory, outlook, overlook, peanut gallery,
   peephole, pharos, platform, ringside, ringside seat, seamark,
   sighthole, top gallery, tower