Besieging Early Modern Fortifications
With the innovations of the early modern era the act of successfully besieging and capturing a fort became increasingly methodical and mathematical in nature. In order to protect troops before reaching the walls, trenches were dug with strong points for defence, as well as artillery batteries to place cannons for bombarding the walls. Once at the base of the walls, artillery batteries were built with sufficient firepower to cause a breach in the walls, through which an assault could take place. This assault needed careful preparation as well as the creation of bridges to allow soldiers to cross the various moats and trenches the defenders had dug. All of this had to take place under the constant threat of the defenders sallying out from the fortification, a common delaying tactic during the trenching phase.
With the continuous increase in firepower that a fort could bring to bear on an opposing army, new tactics had to be devised to reach the base of the walls and to create the possibility of breaching them. . The images above, taken from Petit Traitté de la Fortification moderne, show two things. The first image shows troops digging a trench; these trenches helped to protect the attackers from both artillery and musket fire coming from the fort. The second image depicts a trench cover. These covers were placed periodically along the trenches to create extra protection for both troops and supplies as the trenches stretched towards the base of the walls.
The art of trenching to the base of a fort's wall evolved over time. This depiction from the Petit Traitté de la Fortification moderne, shows how a typical trench system would be built. Trenches would streak out towards the walls with occasional strong points built at intersecting trenches. The strong points allowed troops to defend themselves in the case of a sortie by the defenders. In this image, strong points have a yellow tint. The second type of permanent structure built were artillery batteries. These batteries were placed to both pound the walls of the fortification, as well as protect the troops digging the trenches with accurate counter-fire. In this image, these batteries are the red rectanglular structures.
Once at the foot of the walls a breach needed to be created in order to storm the fort. This was done by concentrated artillery fire from artillery batteries close to the walls. This image from the Petit Traitté de la Fortification moderne shows where such concentrated firepower would be targeted at the courseways between the forts bastions, with the aim of storming a bastion and taking the fort.
When preparing for the final assault of a fort there was often a need to create an object to allow troops to cross over either a moat or deep trench dug around the walls. This image, from the Petit Traitté de la Fortification moderne, shows both the concentrated targeting of a section of the wall with the aim of breaching it, as well as a makeshift bridge at the bottom that could be laid over the moat to allow troops to cross.