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.  But they have many defects.  There is a sector without fire in front of each salient, and the ditches are without protection.  The latter objection also holds good against all circular works.

The redan (Fig. 45) is frequently used to cover a point in rear, as a bridge, a ford, or a defile.  When used alone, its gorge should be closed by palisades.  Its ditches are unprotected.

The lunette (Fig. 46) has nearly the same defects as the redan.

The mitre, or priest-cap, (Fig. 47,) may be employed with advantage when a cross-fire is required on the capital of the work.  The star-fort has all the defects, without the merit of simplicity, which belong to the polygonal redoubt.

The bastion-fort (Fig. 48) more fully satisfies the conditions of a good defence than any other plan; but it is less simple and easy of execution.  It is usually composed of four or five fronts, but it may be applied to a polygon of any number of sides.

For the details of the construction of these several works, we must refer to the special treatises on field-fortification.

Lines of intrenchments may be made either continuous or with intervals.  In adopting either plan, the engineer should avail himself of all the natural obstacles presented by the position, so as to diminish the labor of erecting artificial means of defence.

The simplest arrangement for a continuous intrenchment is the cremailliere or indented line.  When applied to an irregular site, or used to connect together distant and detached works, the indented line may be regarded as a good disposition.  Mitres and redans, connected by straight curtains, are sometimes employed, as also a combination of large and small redans, forming alternate salient and re-entering angles.  A continuous line of bastions is preferable to any other arrangement, when there is plenty of time for their construction.

Lines with intervals are frequently formed of alternate lunettes and square redoubts.  Other detached works may be employed in the same way.  This manner of intrenching a position has several advantages, with disciplined troops.  The first shock of the assailant is sustained by the detached works, and when he attempts to penetrate in the intervals, his flanks become exposed to a deadly cross fire.  These intervals also allow the assailed to act on the offensive, by charging the enemy at the opportune moment.  But with raw and militia forces it will be safer to resort to continuous lines.  If cavalry form any part of the defensive force, it will be absolutely necessary to leave intervals through which these troops may charge.