The Collaborative International Dictionary of English v.0.48:

Fortification \For`ti*fi*ca"tion\, n. [L. fortificatio : cf. F.
   fortification.]
   1. The act of fortifying; the art or science of fortifying
      places in order to defend them against an enemy.
      [1913 Webster]

   2. That which fortifies; especially, a work or works erected
      to defend a place against attack; a fortified place; a
      fortress; a fort; a castle.
      [1913 Webster]

   Fortification agate, Scotch pebble.

   Syn: Fortress; citadel; bulwark. See Fortress.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fortification
    n 1: defensive structure consisting of walls or mounds built
         around a stronghold to strengthen it [syn: fortification,
         munition]
    2: the art or science of strengthening defenses
    3: the addition of an ingredient for the purpose of enrichment
       (as the addition of alcohol to wine or the addition of
       vitamins to food)

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "fortification":
   abatis, advanced work, affirmation, attestation, authentication,
   backing, backing up, balistraria, bank, banquette,
   barbed-wire entanglement, barbican, barricade, barrier, bartizan,
   bastion, battlement, bearing out, bolstering, breastwork, bulwark,
   buttressing, case hardening, casemate, certification,
   cheval-de-frise, circumstantiation, circumvallation, confirmation,
   contravallation, corroboration, corroboratory evidence,
   counterscarp, curtain, demibastion, dike, documentation,
   drawbridge, earthwork, enclosure, enrichment, entanglement, escarp,
   escarpment, fence, fieldwork, fortalice, glacis, hardening,
   invigoration, loophole, lunette, machicolation, mantelet, merlon,
   mound, outwork, palisade, parados, parapet, portcullis,
   postern gate, proof, proving, proving out, rampart, ratification,
   ravelin, redan, redoubt, refreshment, reinforcement,
   reinvigoration, restoration, restrengthening, revivification,
   sally port, scarp, sconce, stockade, strengthening, substantiation,
   support, supporting evidence, tempering, tenaille, undergirding,
   validation, vallation, vallum, verification, vitaminization,
   work