The placing and spacing of a Front of Fortification followed specific geometric formulas. The first stage in tracing out a front began by drawing a polygon within which the whole area of the fortress to be defended was to be contained. The next step was to determine the distance from the salient angle (outermost point) of one bastion to the same point on the adjacent bastions (point A, Fig. 1). This was the measurement from which all the calculations are derived. In this example I have used Vauban's measure of 180 toises, or 360 yards.

A line is drawn between these two points which is then bisected. Then a perpendicular line is drawn through line A - A at the point of intersection (point B) and a point C noted along this line (Fig. 2). The distance from point B to point C was determined by the over-all shape of the fortress.

If the polygon was a hexagon (six sides) the distance was 1/6th of the length of the line A - A. If the polygon was a pentagon (five sides) the distance was 1/7th the length of the length of the line A - A, and 1/8th if the polygon was a square. Next, a line was drawn from point C to point A and at a distance of 2/7th of the length of this line from C to A another point was noted, point D (Fig. 3). The line D to A was the length of the face of the bastion. The distance from D - C was also the distance from D to E which gives the length of the flank wall of the bastion. The line of the curtain wall was now drawn in by joining E to E.


Fig. 4 shows the completed front. The area at the level of the surrounding country is taken as the DATUM. The height of any construction above this is its COMMAND. In the rear of the rampart is a raised level, the TERREPLEIN, upon which cannon could be mounted. Then comes the BANQUETTE, a fire-step to enable the defending infantry to fire over the PARAPET. The rampart drops away from the crest of the parapet in two sections, the upper section being the SUPERIOR SLOPE and the lower section that runs down to the level of the datum is the EXTERIOR SLOPE. The inner wall of the ditch is the ESCARP and the outer wall is the COUNTERSCARP. Before the glacis runs the COVERED WAY which permitted troops to form for defence or for an attack upon the lines of the besieger.

Jumbo Figs 4 and 5 (slow: 70K)

From this basic front numerous complex and sophisticated designs were developed. The emphasis was always upon providing opportunities for flanking fire to be delivered from every possible angle upon every front that might be attacked. Beyond the main ditch further defensive constructions were added, each angled to present the minimum possible target to the enemy whilst allowing every facility for enfilading an attacker.

In Fig. 5, RAVELINS have been added to cover the curtain and the flank of the bastion as well as to create an additional obstacle before the main work. The construction of the ravelin begins at point F where the counterscarp makes its re-entering angle in the centre of the face along the perpendicular line B - C. This line is extended outwards to point G which is 110 yards in front of F. From this point lines are drawn back towards the shoulders of the bastions (point D). These lines form the flanks of the ravelin which end at the line of the counterscarp. The ditch is then extended around the new front.

In Fig. 6, a COUNTERGUARD has been placed ahead of the bastion. A REDOUBT has been built within one of the ravelins and TENAILLES of differing types have been constructed in the ditch to protect the curtain and to flank the ditch. The rear face of the counterguard (H) is based on the counterscarp wall and the salient angle of the adjacent bastions (point A). The tenailles were set some 2 or 3 feet higher than the body of the ravellin to enable its defenders to fire into the ravellin should that work be taken by the enemy.

Fig. 7 displays the completed front, with one ravelin protected by two TENAILLONS and the other covered by a BONETTE and two flanking LUNETTES. Not only did these outworks protect the ravelin and deter the attacker but they formed a self-supporting and inter-linking complex of defences, all with ditches that had to be taken by assault before the main works could be approached. The lunettes are constructed by bisecting the face of the ravelin and extending the line 60 yards outwards from the counterscarp of the ravelin ditch to point I. The outer face is found by extending the ditch to point J. The rear face of the work is the counterscarp of the main ditch. The bonette covers the flanked angle of the ravelin, its rear face being on the counterscarp and its faces being produced by bisecting the face of the lunettes. The tenaillons are plotted by extending the faces of the ravelin across the ditch for 60 yards from point K to point L and by measuring 30 yards along the counterscarp of the main ditch from point M to N.

Other, much larger independent and semi-independent works, HORNWORKS and CROWNWORKS, were often constructed ahead of the main enceinte. As these were expensive both to build, man and arm, they were usually only situated at particularly important points in the defences. By enclosing the rear face of the work it could become a completely detached outwork that could be placed on important pieces of ground beyond the fortress's principal lines.

Both hornworks and crownworks can be constructed in front of either bastions or curtains. When a hornwork is built before a curtain it is laid out by plotting a point ( O ) 160 yards in front of the salient angle of the ravelin (G ). From this point a line at right angles is drawn, noting two points (P) at 110 yards to each side. Using these two points, a front of fortification is developed in the same way as for the main body of the fortress, with a face (P - Q) 60 yards in length and the perpendicular, 36 yards. The flank is drawn as far as the counterscarp of the main ditch. The work is completed by a ravelin (R) 70 yards deep whose faces are aligned with the point Q. (Fig. 8)

To construct a crownwork before a bastion, a point ( T ) 240 yards from the flanked angle of the bastion is drawn and from this point the two fronts of the crownwork are plotted using the same dimensions of 60 yards for the faces (U - V) and 36 yards for the perpendicular (V - W) as in the hornwork. The flanks are produced from U to terminate opposite the adjacent ravelins, 50 yards in from the main ditch. The work is completed with ravelins, again using the same calculations as for the hornwork. (Fig. 9)

Fig 10: Plan of a citadel, showing some of the features described as well as some of the variants.

With the use of the above information, the reader can identify the principal structural features of almost every permanent fortification that was in use throughout the Napoleonic Wars (see Fig. 10). It must, however, be mentioned that not every aspect of fortification has been described here and that there were many variations from the usual configurations, though in almost all circumstances the same basic calculations were adopted. It should also be borne in mind that some fortresses dated from medieval times and were not completely modernised, receiving only a few essential improvements. In such circumstances the enceinte rarely forms any kind of polygon.

Design and Contruction of Fortresses Brief Analysis by John Grehan

Introduction and Development
Construction
Glossary