Source : Webster's Revised Unabridged Dictionary (1913)

Fieldwork \Field"work`\, n. (Mil.)
   Any temporary fortification thrown up by an army in the
   field; -- commonly in the plural.

         All works which do not come under the head of permanent
         fortification are called fieldworks.     --Wilhelm.

Source : WordNet®

fieldwork
     n : a temporary fortification built by troops in the field