Civil War Field Fortifications

III. Siege Works

Siege Works were specialized field works necessary for a besieging army to approach a fortified place under cover, breach its successive lines of defensive works, expose its interior spaces to assault, and bring about its capitulation or capture by storming. The various types of siege works used during the American Civil War were fundamentally the same as those developed during the great age of fortress warfare on the European continent in Seventeenth and Eighteenth centuries; Sebastien LePrestre de Vauban plopped anachronistically into the trenches at Vicksburg would have found the Federal army's siege works all too familiar. The great engineer might have been moderately surprised to discover the nature of the thing being sieged: a strong line of field fortifications occupied by a relatively large field army. There were no great fortress cities in America and even in Europe the old fashioned stone and masonry fortresses had declined in importance during the first half of the Nineteenth century while the importance of field fortifications to the defense of strategic points increased. When the American army invaded central Mexico it was confronted by a series of positions covered by field fortifications; during the Crimean War the Russians based their defense of Sebastopol on a line of strong field works that confounded the besieging allied armies. This international trend toward the use of field fortifications to protect strategic points was continued and further developed through the Civil War. When defensive lines were well designed and moderately well constructed they imparted an endurance to the defense that was, at the very least, extremely difficult for an attacking army to overcome by simple assault.

Whatever the trend of the time might have been, the principles of fortification remained the same whether the works were made of stone, brick, or dirt and the necessity of approaching lines of defensive works under cover was just as compelling as it ever had been. An attacking army still required offensive field works to be able to get at the enemy and breach his defenses. Offensive field works could only be used effectively if they were used systematically and methodically, that is, if the attacking army conducted a siege by regular approaches. The methodology of siege warfare entailed an attack conducted in successive and distinct phases that carried the attacking army ever closer to the enemy's fortifications. More to the point here, each phase required specific types of field works to accomplish its objective so that the next phase and the next after that could begin until the fortifications were breached and the fortified place captured.

The following section outlines the successive phases of a theoretical siege of a fortified place. It follows the pattern of examination generally employed in pre-Civil War manuals to introduce the subject to students of the art of war. The outline form unfortunately tends to make the thing seem a great deal easier than it ever could have been in practice, but it must be remembered that the outline merely presents the basic methodology of siege warfare without going into the gory details of the progress of an actual siege. Since the purpose here is to outline the forms of field works necessary to accomplish each phase of a siege in the context of a siege as whole, links have been provided to pages that describe and discuss the various individual types of siege works.

 Phases of The Conduct of an Attack By Regular Approaches

I. Investment and Lines of Circumvallation and Countervallation

As soon as the decision to lay siege to a fortified place had been made the attacking army detached strong advanced forces to unexpectedly severe the fortress' routes of communication and seize any points in the immediate vicinity of the place that the garrison might use to resist the attacking army outside their fortifications. When the main body of the army arrived it fanned out to surround the fortification and solidify its isolation from outside assistance. Lines of circumvallation were constructed to prevent enemy field armies from interfering with the siege and lines of countervallation were established to enclose the garrison with a strong line of field works.

II. Evaluation of the Defensive Characteristics of the Fortified Place and Choice of a Point of Attack

When the investment was completed the engineers, in company with other officers and an escort, if needed, conducted a close examination of the enemy's fortifications and drew up plans of the enemy's works and maps of the ground between the lines of countervallation and the enemy's works. This assessment of the fortification was used to determine which points of the fortification were most vulnerable and would allow the besieging army to conduct an approach by regular siege works in the most economical manner possible. Three important considerations were supposed to influence the choice of a point of attack: strength of the enemy's fortifications, the ground through which an approach would be forced to pass, and the availability of materials necessary to construct the siege works. In general the most vulnerable points in a fortification were its salient angles where the ground in front of the fortification was not covered by direct fire from its parapet.  After the point of attack had been chosen, the approaches and all of the siege works necessary to accomplish the objective of breaching the fortification and bringing about the garrison's capitulation were planned and designed. In the meantime the engineers and sappers started collecting siege materials such as gabions, fascines, sap-fagots, and all the necessary tools, at specially designated depots near the rear of the approaches in preparation for the opening of the trenches. 

III. Laying Out and Opening the First Parallel

As soon as sufficient siege materials had been gathered the engineers prepared to open the trenches by marking the line of the First Parallel on the ground where it was to be constructed. This was accomplished in two steps: first the prolongations of the fortification's columns of fire were marked using small stakes, then the parallel and boyaux leading to it were marked with stakes connected by string or tape. Marking the enemy's columns of fire helped the engineers in charge of the work be sure that no part of the parallel would present a flank or inappropriate salient angle to be enfiladed by the fortification. On the evening after the line was marked working parties were sent forward to begin constructing the First Parallel.  Unless the enemy's artillery fire was particularly bothersome and the working party had to gain cover quickly, the First Parallel was constructed by means of the Simple Sap. By the morning of the next day the First Parallel would be deep enough to provide protection for a second relief of workmen who would continue to work throughout the day to finish the parallel. Boyaux were also started to the rear to provide cover for working parties passing to and from the parallel and depot where the siege materials where gathered.

IV. The Second Parallel

When the First Parallel was nearing completion boyaux were run forward toward the position of the Second Parallel, usually about 300 yards from the salient of the point of attack. These boyaux were generally constructed by means of the simple sap, with the parapet thrown up on the side nearest the point of attack, and were given either a zig-zag pattern or interior traverses to defilade them from enemy fire. When the boyaux reached the position of the Second Parallel, the engineers marked the line of the parallel on the ground, under cover of darkness if necessary, and materials were brought forward from the depot to begin constructing the work. Because this parallel was well within range of enemy canister fire the working parties had to act quickly to gain the cover of the work. To do this the Second Parallel was constructed by means of the flying sap. 

V. Batteries

Before the siege works could be pushed forward from the Second Parallel batteries had to be established to begin the steady and difficult work of suppressing the enemy's artillery fire. Batteries could either be established in front of the First Parallel or in front of the Second Parallel, or both. And could be constructed either as part of the parallel itself, by expanding the width of the trench, or as separate works reached by boyaux leading from the parallels. Batteries placed in front of the First Parallel batteries would be well placed to deliver ricochet fire to dismount the enemy's artillery; those in front of the Second Parallel would have a more certain aim. In either case, batteries were established at points  along the parallels where their parapets would be perpendicular to the faces of the fortification forming the salient angle of the point of attack and thus obtain an enfilade fire against the point of attack. These batteries were armed with a combination of guns, howitzers, and mortars: the guns would fire to break down traverses and dismount the enemy's artillery, the howitzers would deliver their shells to harass enemy troops in the covered way and along the parapet of the fortification, while the mortars lobbed their shells into the interior of the fortification.

Note: Details on the construction of batteries can be found under the heading Batteries in the Minor Works section.

VI. The Third Parallel

Beyond the Second Parallel the enemy's musket fire was assumed to began to take effect on the head of the approach trenches; small arms fire was more constant and could be better directed and therefore was considered much more dangerous to the troops working the trenches than the enemy's artillery fire. The greater danger involved in pushing the approaches beyond the Second Parallel required greater protection for the engineer troops, the specialist sappers, who would be assigned the task of constructing the boyaux leading to the position of the Third Parallel. This protection was provided by constructing the boyaux by means of the full sap. This was a specialized operation that required skill and experience to accomplish with the least amount of exposure while maintaining a steady rate of advance. Covered in front by a heavy sap-roller and protected from enemy sorties by trench guards to the rear, the sappers slowly worked the full sap to the line of the Third Parallel. Demi-parallels intended to provide defensive works for the trench guards  were branched to the right and left from the boyaux. When the the sappers finally dug their way to the position of the Third Parallel, usually about 60 yards from the salient of the point of attack, they branched the trench to the right and left by means of the full or half full sap to form the parallel. In the unlikely event that the enemy's fire was not very heavy the parallel could be constructed, like the Second Parallel, by means of the flying sap.

Mining operations were usually started at the Third Parallel. If the enemy had an extensive network of counter mines, these had to be defeated by mining operations before the main sap could be run forward. When the attackers intended to sink their own mine under the rampart of the fortification the mine galleries were started from the Third Parallel. Either way, mining operations tended to delay the advance of the sap due to the risk to the sappers and their work from enemy mine explosions. Resourceful garrisons could delay the progress of the besieging army's approaches almost indefinitely by using extensive (and lucky) counter mining operations.

VII. Stone Mortar Batteries and the Trench Cavalier

After the Third Parallel was completed the immediate object of the attack was to force the enemy to abandoned the covered way.  This could either be accomplished by assault or by continuing the regular approach. Assaults were frequently unsuccessful and even when successful resulted in heavy losses that were difficult for the besieging army to withstand or justify.  The preferred method was to continue the approach. To do this stone mortar batteries, which did not require embrasures, were excavated in front of the Third Parallel where they could lob their wads of stones into the covered way. The full sap was again run forward under cover of the mortar fire to a point about 30 yards from the crest of the glacis. At that point secondary trenches were branched to the right and left parallel to the covered way and extended 15 to 20 twenty yards from the main sap. Working parties under direction of the sappers or engineers would then construct the  massive trench cavaliers by throwing up a parapet about ten feet high or high enough for troops manning its banquette to obtain a plunging fire into the covered way. Fire from the trench cavaliers combined with that from the small mortars would eventually compel the defenders to abandon the covered way and allow the sappers to run the main sap forward to crown the glacis.

VII. Crowning the Glacis and Establishing the Breaching Batteries

When the sap reached a point about six yards from the crest of the glacis the sappers extended it to the right and left to envelop the covered way.  Breaching batteries were then constructed along the crest of the glacis in such a way that the guns could direct their fire down on the lower portions of the scarp wall and bring about its collapse, thus opening a breach in the fortification's rampart. Howitzers would then fire into the collapsed debris to break it down until it formed a practicable ramp that assaulting troops could easily surmount.

VII. Crossing the Ditch and the Lodgment of the Breach

At this point many garrisons were willing to admit defeat and ask for terms of capitulation, others chose to risk at least one assault before asking for terms. Occasionally a well commanded garrison composed of good troops would fight on from behind their retrenchments. If an assault on the breach was unsuccessful or undesirable due to the heavy losses necessarily involved, the besieging force would be compelled to descend into the ditch and run the full sap into the breach. A gallery, either constructed as a mine or by using cross traverses and blinds above the sap, was pushed forward from the glacis to the bottom of the ditch. The bottom of the ditch was crossed by means of the full sap and the trench then pushed on up into the breach. A substantial defensive work prepared for musket and artillery fire was then constructed across the breach and operations were started to breach the enemy's interior retrenchments.

Only the most desperate of garrisons refused to surrender after the last line of works was breached; most had the good sense to ask for terms before the besieging army was forced to attempt an assault on the last barrier. The customary alternative to capitulation was the relentless massacre of both the garrison and the civilian population unfortunate enough to be trapped in the fortified place if it had to be captured by storming.

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