Fortification Friday: Plans for simple “intrenchments” – Part 1, Open Works

Field fortifications share many components with the permanent fortifications.  All fortifications feature faces, flanks, curtains, and salients.  Much of what we’ve discussed with regard to traces and profiles fit in the lesson plans for both any type of fortification.  But field fortifications, unlike the permanent type, usually began as entrenchments… or as Mahan called them in his time, intrenchments.  Often several detached entrenchments would evolve into a larger set of works, thence labeled a fortification.  So from the perspective of planning these field fortifications, one could use one of several basic, or simple, forms of entrenchments.

Mahan discussed these “plans of simple intrenchments” more so as examples for the engineer to adapt as needed to the situation.  And simple was the watch word:

A great variety of figures has been used for the plan of simple intrenchemnts.  They may all be reduced to the following, the Right Line; the Redan; the Lunette, or Detached Bastion; the Crémaillère, or Indented Line; the Priest-Cap, or Swallow-Tail; the Redoubt; the Star Fort; and Bastion Fort.

From the start, we see that Mahan sought to avoid confusion with a more nuanced discussion of possible varieties.  Instead we see eight basic forms to be repeated and refined.  Over the next few installments, I’ll detail each of these in turn.  But first, let us look at those at the start of Mahan’s list, which represented open works.   This is “open” in the sense that the trace did not represent a closed polygon.

The right line was the basic of these entrenchments and was the basis for the others, in terms of evolution.  Mahan did not elaborate on the right line in his discussion of field fortifications.  This was, however, the basic line which we’ve discussed in regard to the plans before.  A right line is one which delivers direct fire.  Recall that direct fire is perpendicular to the (expected) enemy line, and thus is defined by a right angle… thus the name right line.


The problem with a right line is that by itself there is no flank defense.  So the first evolution would be to add a work that could provide enfilading fires.  One basic addition was called a redan:

The redan is a work consisting of two faces; the gorge, or entrance in the rear, being open.  This work is used to cover a point in its rear; such as a bridge, defile, ford, &c.

The figure provided to illustrate the redan:


For the moment, disregard the “dashed” portion at the tip of the angle.  Notice that a redan is really nothing more than a salient detached from any surrounding work.  Though, that was not always the case, nor was it common to have a completely detached redan.  Usually additional trenches… er… right lines… extended around the redan to connect it with other works.  The redan’s main difference compared with salients in forts is its lack of rear protection. Mentioned in Mahan’s definition, line A-D is the gorge.  As you can see, there is no gorge wall. Again, this is an “open” work.

Also note, from the figure, the preferred salient angle for a redan was 60º.  And the preferred length of a redan’s face was 30 to 60 yards. These relate back to the principles regulating the defense. Redans were useful, as Mahan stated, to cover an important point.  Such an entrenchment provided depth to the defense.  Another common place to employ redans was to reinforce a long right line.  By introducing enfilading fires, the enemy had to adjust their line of attack further back than when simply opposing a right line.

But redans, being salients, had their flaws:

Having no flank defenses its salient is unprotected, and to obtain a fire in the direction of its capital a short face, denominated a pan coupé, is sometimes made in its salient angle.

This is where the dashed part at the tip of the angle is the pan coupé, along line B-C.   Furthermore, keep in mind the pan coupé was but one variation that could address the weak points of the redan.  Other options included auxiliary flanks, flanks with faces, along with double or even triple redans.  But … the word of the day is “simple.”  So let us save those for another day.

A more elaborate evolution off the right line was the lunette:

The lunette consists of two faces and two flanks. This work is used for the same purposes as the preceding.  IT has the same defects; but possesses the advantages of sweeping with fire of its flanks ground which might be badly defended by its faces.

These features are demonstrated by figure four:


Lines B-C and C-D are the faces.  Lines A-B and E-D are flanks.  Line A-E is the open gorge.  The faces were to be 46 to 60 yards in length, compared to the faces which ran 20 to 40 yards.  Note the arrows indicating the line of fire from points around the lunette. Lunettes, like redans, added depth to the defensive arrangements.

There is some ambiguity with use of the term lunette.  In fixed fortifications, the lunette was an auxiliary feature to support a demi-lune or other advanced work.  Likewise, we often hear of single artillery positions called lunettes.   The word itself derived from the French for “little moon” and came to describe many half-moon shaped features in art and architecture, in addition to its use in military science.  So naturally the half-moon shaped revetments used for artillery came to be called lunettes… even though they looked little like Mahan’s lunette.

While the redan and lunette might be variations on a theme, the indented line (I can’t spell Crémaillère with consistency) was somewhat a different animal:

The indented line serves to convert the direct fire of a right line into a flank and cross fire, and therefore frequently substituted for the right line.

Figure five illustrated the indented line:


What we see here is a series of faces and flanks arranged to provide parallel angles of defense, and thus a useful cross fire.  All angles in the line were 90º.  The suggested length of the face (A-B) was 60 yards, while the flanks (B-C) were 20 yards. Indented lines worked best when placed on commanding terrain.  But while generally strong, the direction of fire from these lines was somewhat rigid.  Any variation in the terrain required some adjustment of the angles and thus the line.  So it looked great on paper… but might not work when walked out on the field.

The last of these “simple” open works was the priest-cap or swallow-tail, as you prefer:

The priest-cap is seldom used as a detached work; but is generally combined with the right line and the indented line to procure a flank, or cross fire, in front of them.

The general arrangement looked thus:


Lines A-B and C-D were faces, running 60 to 100 yards.  Lines B-E and C-E were reentering lines some 30 yards long.  And we have an open gorge between A and D.  Notice the angles involved to complete this – a 90º at point E to create the reentering angle.  This left two salients at points B and C which were 60º, preferred that is.

Mahan mentioned the best employment for a priest-cap.  I would add that this form required the most effort compared to the other open works.  And that is relative, all things considered.  As we turn to the enclosed works, keep in mind the effort required to fill in the gorge line in order to create the polygonal form.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 11-12.)


Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

5 thoughts on “Fortification Friday: Plans for simple “intrenchments” – Part 1, Open Works

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