A fieldwork is a temporary structure, built for defensive or offensive use during military operations and designed to protect infantry and often to act as a gun emplacement also. They were built of earth, which may have been reinforced with timber revetting; the defences may sometimes have been strengthened with palisades etc. Those with a defensive function were sited to protect a feature such as a town, village, or their approaches, and those with an offensive function to dominate as far as possible the defended positions and to contain the besieged area within a "line of circumvallation".
All fieldworks are formed of banks and ditches, but there was great variation in their layout, the simplest form being a breastwork, ie a simple parapet of earth, and the most complex being a fort or sconce, - a rectangular enclosure with bastions at each corner and with flanking defences and other features. Large sites such as towns may have a continuous bastioned enceinte, a continuous line of defence of banks and ditches encircling the site and supported by bastions and outworks as at Newark, Nottinghamshire. The defences may be encircled by lines of circumvallation, together with lines of approach towards the defended area, as at Newark again, where the besieging forces also fortified villages in the locality. They will be recognised in the field as earthworks or sometimes as cropmark/soilmark sites; they may also be shown on maps, and are sometimes given names (Queen's Sconce, Oliver's Battery, etc) but these should be treated with caution.
Sites that may be confused with this class are other earthworks such as moated sites and dams (Taylor 1974, 21, 34) and more recent features such as 20th century wartime gun emplacements; the siting of these features will be a significant pointer to their role.
Excluded from this description are mottes built as siege works (or as adulterine castles), permanent earthen artillery forts, and 20th century gun emplacements, which are all considered as separate classes.
Fieldworks were designed to defend a particular feature or features, or to provide a protected base from which the defended areas could be attacked by sapping, infantry, and gunfire.