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Sapping is a term used in siege operations. Any trench excavated under defensive musket or artillery fire that was intended to advance a besieging army's position in relation to the works of an attacked fortification was referred to as a sap. Saps of approach were excavated by brigades of trained soldiers, often called sappers, because they dug the saps, or specifically instructed troops of the line.
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By using the sap (trench) the besiegers could move closer to the walls of a fortress, without exposing the sappers to direct fire from the defending force. To protect the sappers, trenches were usually dug at an angle in zig-zagged pattern (to protect against enfilading fire from the defenders) and at the head of the sap a defensive shield made of gabions (or a mantlet) could be deployed.
Once the saps were close enough, siege engines or cannons could be deployed to batter a breach in the curtain walls. Prior to the invention of large pieces of siege artillery, miners could start to tunnel from the head of a sap to undermine the walls. A fire or gunpowder would then be used to create a crater into which a section of the fortifications would fall creating a breach.
The Age of Gunpowder
During Cologne War, in the Siege of Godesberg of 1583, even the large caliber cannons brought to bear on the fortress by the superior force of Ferdinand of Bavaria had little impact on the fortress itself. The cannons were firing heavy shells, but the height of the fortress significantly reduced the power with which they could hit the walls. Although the fortress dated from the 14th century, its construction caused the cannonballs to "bounce" from the walls, having little impact. To breach the walls, Ferdinand ordered his soldiers to dig into the feldspar supporting the side of the mountain. Even when the powder was ignited and a substantial portion of the wall, the gate, and the inner walls were blown into the air, the defenders still held out for three days.
Trace Italienne
Sapping became a necessary process after the development of the Italian style bastion, or trace Italienne, in defensive architecture that made siege warfare the modus operandi of military operations in the late medieval and first decades of the early modern period of warfare. Fortresses boasting abutments with gentler angles were difficult to breach; cannonballs and mortar shells often had little impact on the walls, or impact that could be readily repaired after night fell. Towers no longer protruded at right angles from the wall; rather, they blended with the wall. These created a two-fold advantage. First, defenders in the towers had a field of fire of 280 degrees or more. This range of fire and the towers' positioning allowed them to fire upon the attackers' flank as they advanced, a deadly fire called enfilade. Consequently, a seco which range their cannons were less effective.
17th Century
During the English Civil War there was a siege of Newark-on-Trent which took place from 6 March 1645 – 8 May 1646. A detailed map of the Cavaliers defences of Newark and the lines of circumvallation and contravallation along with the besiegers redoubts and fortified camps was drawn up by R Clampe, the besieging Roundheads' chief engineer. It includes a zig-zag sap emerging from a bastion of the circumvallation. The zig-zags are at such angles and positions that the defenders were unable to bring enfilade fire to bear. Once the sap was completed four cannons were placed much closer to a gateway than those in bastions of the circumvallation.
American Civil War
In the American Civil War, troops advanced their sap under cover of a sap roller or mantlet by forming a parapet on the engaged side of the trench one gabion at a time and filling it with earth taken from the trench.
Russian sap
The term Russian sap refers to a tunnel that is dug at a shallow depth under no man's land towards an enemy position. It allows the attacking infantry to approach the enemy position beneath the surface of the earth, without being detected and safe from enemy fire. For the attack, the tunnel is opened and the infantry attacks the enemy position at comparatively short range. Russian saps were widely used in the First World War, for example during the Battle of the Somme, when four of them were further equipped with Livens Large Gallery Flame Projectors.