The Collaborative International Dictionary of English v.0.48:

Blockhouse \Block"house`\, n. [Block + house: cf. G. blockhaus.]
   1. (Mil.) An edifice or structure of heavy timbers or logs
      for military defense, having its sides loopholed for
      musketry, and often an upper story projecting over the
      lower, or so placed upon it as to have its sides make an
      angle wit the sides of the lower story, thus enabling the
      defenders to fire downward, and in all directions; --
      formerly much used in America and Germany.
      [1913 Webster]

   2. A house of squared logs. [West. & South. U. S.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

blockhouse
    n 1: a stronghold that is reinforced for protection from enemy
         fire; with apertures for defensive fire

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "blockhouse":
   acropolis, bastion, beachhead, box, bridgehead, bungalow, bunker,
   cabin, castle, chalet, citadel, cot, cote, cottage, donjon,
   fasthold, fastness, fort, fortress, garrison, garrison house, hold,
   keep, lodge, log cabin, love nest, martello, martello tower, mote,
   motte, peel, peel tower, pied-a-terre, pillbox, post, rath,
   safehold, snuggery, strong point, stronghold, tower,
   tower of strength, ward