Temporary fortifications, as a part of field-engineering, may therefore be regarded rather as an _arm_ than an _art_.

The principles of their construction are derived, of course, from the theory of permanent fortification, but in applying these principles to practice in the field, much greater latitude is allowed than in the exact scientific arrangement of permanent works.
The purpose of field-works (or intrenchments, as they are commonly called) is to arrest, or at least to impede, the march of the attacking foe; to shelter the defensive troops from the missive weapons of the assailants, and to detain them in a position where they will be exposed to the fire of the defensive force.

The numerical and positive strength of the assailed may be much less than that of the assailant, and yet an equilibrium exist; the material obstacles compensating for the difference in numbers.

Intrenchments, though inert masses, must therefore be regarded as most valuable and important accessaries in the defence of a position.
Intrenchments consist either of _lines_ of works made to cover extended positions, or of _detached_ works designed simply to defend the ground they occupy.

The former generally present a front against the enemy in but one direction, while the latter are usually closed on all their sides.
The following figures have been employed for the plan of simple intrenchments, viz.: the polygon, redan, lunette, mitre, star-fort, and bastion.
_Square_ or _polygonal redoubts_ are the most common forms given to field-works, on account of the ease of their construction.