The Collaborative International Dictionary of English v.0.48:
Fieldwork \Field"work`\, n. (Mil.) Any temporary fortification thrown up by an army in the field; -- commonly in the plural. [1913 Webster] All works which do not come under the head of permanent fortification are called fieldworks. --Wilhelm. [1913 Webster]WordNet (r) 3.0 (2006):
fieldwork n 1: a temporary fortification built by troops in the field