Fortification Friday: Resolving the inherent defect of a bastion fort

Sorry for skipping an installment last week.  But I’ll try to make up for that this Friday.  So let us continue the discussion of Mahan’s preferred field fortification – the bastion fort.  We walked through the method to lay out these sort of defenses, noting the geometry involved.  In with that method, there are some terms we should set forward, as they factor into the detailed discussion of such works:

The side of the polygon is termed the exterior side; the line bisecting it, the perpendicular; the angle at the salient is the flanked angle; the one formed by a face and flank, the shoulder angle; the one between the flank and curtain, the angle of the curtain; the portion of the work included between the capitals of the two adjacent bastions is denominated a bastioned front, or simply a front.

Lots of terms, and sharp readers will notice many of these overlap terms we’ve discussed before.  Let’s walk through these, starting with the exterior side, were we can reuse one of the earlier illustrations:

BuildingBastionPt1

This defines the “workspace” the engineer hopes to use for the fortification.  The works will fit inside this polygon defined by the exterior sides.  It also orients, for functional analysis, the fortification’s defenses.  Being that the exterior side is the facing from which the “bad guy” is going to be engaged.  But keep in mind the exterior side is more an “imaginary” line as opposed to a line that will be constructed.

The same “imaginary” status applies to the perpendicular:

BuildingBastionPt2

This line might also be considered as orienting the primary direction of resistance from the defender’s perspective.  Granted, that resistance would come in the form of flanking fires from the bastions and thus not define the line of fires.

The rest of these features are less so “imaginary” and were structures requiring the pick and shovel. Within the context of a bastion fort, the salient angle was called the flanked angle:

BastionFlankedAngle

Very easy to think of the shoulder angles as those supporting the weight of the defense against the adversary:

BastionShoulderAngle

The re-entering angles are, in context of the bastion fort, the angles of the curtain:

BastionCurtainAngle

And taking into consideration the entire arrangement of angles, faces, and flanks, we have the bastioned front:

BastionFront

Notice how this complements the orientation mentioned that was defined by the exterior side.  You get the impression the bastion fort was darn near impregnable?  Well maybe not impregnable but at least very difficult to assail, right?  Well, Mahan was quick to point out that, for all its good qualities, the bastion fort had a weak point that must be addressed:

An examination of the arrangement of a bastioned fort will show that there are neither dead angles nor sectors without fire; that the salients, and all the ground within the range of musketry, are protected by formidable columns of direct, flank, and cross fire. There is one point in this system that demands particular attention, which is, that the counterscarp of the ditch, if laid out parallel to the interior crest, would form a dead angle along each face near the shoulder; because the fire of the flank would be intercepted by the crest of the counterscarp.

Let’s look at that against a diagram of a bastioned front:

BastionWeakSpot

The rectangles depict the three fields of fire we are interested in here – that of the bastion’s face, the bastion’s flank, and the face opposite the bastion.  Notice how at the shoulder angle, there is a sector without fire.  That sector is mostly covered by the opposite flank.  However, at the very apex of the sector without fire’s angle, there is a small spot where no coverage exists.  What makes that point even more dangerous is, when considered from the profile view, the weak spot is covered by the counterscarp of the ditch:

BastionWeakSpot2

Of course, the enemy knowing this to be a natural flaw in a bastioned front, would be sure to target that spot in their advance.

So how to solve it?  Mahan offered two methods:

To prevent this, either the counterscarps of the faces must be prolonged to intersect,and all earth between them and the scarp of the flanks and curtain be excavated, or the ditch of each face must be inclined up in a slope from the bottom, opposite the shoulder, so that it can be swept by the fire of the flank.

Let us consider the first option, Figure 13 for Mahan’s second plate:

BastionFixOption1

Basically, a lot more digging!  Mahan added, “The first method is best, but requires most labor…”

The second method was illustrated by Figure 14 of the same plate:

PlateIIFig14

In short, for the second method, we see the ditch in front of the faces being sloped lower and lower the further it was from the opposite flank. You can see that in the profile added in Figure 14, above.   And even that remedy came with some issues.  Mahan wrote

… the second is chiefly objectionable as it gives an easy access to the ditch, which might be taken advantage of in an assault.  It is proposed, to obviate this, to dig a second ditch at the foot of the slope across the main ditch, twelve feet wide, and about six feet deep; to make it pointed at the bottom, and to plant a row of palisades in it.

Though not requiring as much labor as the first option, the second option added work to the detail.  Furthermore, the need to maintain the contours of the feature added to the upkeep of the works.

Overall, we can say the bastion fort, even with this weak spot, was still preferable to the other simple entrenchment forms.  Yet, we also see that these works had to be planned, constructed, and maintained to exacting standards, lest all that effort be undone by an enemy’s carefully executed assault.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 14-5.)

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