A polygonal fort is a type of fortification originating in France in the late 18th century and fully developed in Germany in the first half of the 19th century. Unlike earlier forts, polygonal forts had no bastions, which had proven to be vulnerable. As part of ring fortresses, polygonal forts were generally arranged in a ring around the place they were intended to protect, so that each fort could support its neighbours. The concept of the polygonal fort proved to be adaptable to improvements in the artillery which might be used against them, and they continued to be built and rebuilt well into the 20th century.
Bastion system deficiencies
The bastion system of fortification had dominated military thinking since its introduction in 16th century Italy, until the first decades of the 19th century. The French engineer Sébastien Le Prestre de Vauban also devised an effective method to defeat them. Before Vauban, besiegers had driven a sap towards the fort until they reached the glacis, where artillery could be positioned to fire directly on the scarp wall to make a breach. Vauban used saps to create three successive lines of entrenchments surrounding the fort, known as "parallels". The first two parallels reduced the vulnerability of the sapping work to a sally by the defenders, while the third parallel allowed the besiegers to launch their attack from any point along its circumference. The final refinement devised by Vauban was first used at the Siege of Ath in 1697, when he placed his artillery in the third (innermost) parallel at a point close to the bastions, from where they could ricochet their shot along the inside of the parapet, dismounting the enemy guns and killing the defenders. Other European engineers quickly adopted the three-parallel Vauban system, which became the standard method and would prove to be almost infallible. Vauban designed three systems of fortification, each having a more elaborate system of outworks, which were intended to prevent the besiegers from enfilading the bastions. During the next century, other engineers tried and failed to perfect the bastion system to nullify the Vauban type of attack. During the 18th century, it was found that the continuous enceinte, or main defensive enclosure of a bastion fortress, could not be made large enough to accommodate the enormous field armies which were increasingly being employed in Europe; neither could the defences be constructed far enough away from the fortress town to protect the inhabitants from bombardment by the besiegers, the range of whose guns was steadily increasing as better manufactured weapons were introduced.
Theories of Montalembert and Carnot
Marc René, marquis de Montalembert (1714–1800) envisaged a system to prevent an opponent from establishing their parallel entrenchments by an overwhelming artillery barrage from a large number of guns, which were to be protected from return fire. The elements of his system were the replacement of bastions with tenailles, resulting in a defensive line with a zigzag plan, allowing for the maximum number of guns to be brought to bear and the provision of gun towers or redoubts (small forts), forward of the main line, each mounting a powerful artillery battery. All the guns were to be mounted in multi-storey masonry casemates, vaulted chambers built into the ramparts of the forts. Defence of the ditches was to be by caponiers, covered galleries projecting into the ditch with numerous loopholes for small arms, compensating for the loss of the bastions with their flanking fire. Montalembert argued that the three elements, would provide long-range offensive fire from the casemated main curtain, defence in depth from the detached forts or towers and close-in defence from the caponiers. Montalembert described his theories in an eleven-volume work called La Fortification Perpendiculaire, which was published in Paris between 1776 and 1778. He summarised the benefits of his system thus; "...all is exposed to the fire of the besieged, which is everywhere superior to that of the besieger, and the latter cannot advance a step without being hit from all sides". A full realisation of Montalembert's ambitious plans for a great inland fortress was never attempted. Almost immediately after publication, unofficial translations into German were being made of Montalembert's work and were being circulated amongst the officers of the Prussian Army. In 1780, Gerhard von Scharnhorst, a Hanoverian officer who went on to reform the Prussian Army, wrote that "All foreign experts in military and engineering affairs hail Montalembert's work as the most intelligent and distinguished achievement in fortification over the last hundred years. Things are very different in France". The conservative French military establishment was wedded to the principles laid down by Vauban and improvements made by his later followers, Louis de Cormontaigne and Charles Louis de Fourcroy. What little political influence the aristocratic Montalembert had during the Ancien Régime was lost following the French Revolution in 1792.
Despite the conservatism of the French engineer corps, two French engineers experimented on a modest scale with Montalembert's ideas for detached forts. Jean Le Michaud d'Arçon, ironically one of Montalembert's detractors, designed and built a number of lunettes (an outwork resembling a detached bastion) which were in accord with Montalembert's concepts. These lunettes were constructed at Mont-Dauphin, Besançon, Perpignan and other border fortresses, commencing in 1791 shortly before the Revolution. In the same year, Antoine Étienne de Tousard took up a position on Malta as an engineer to the Order of Saint John and was instructed to design a small fort to command the entrance to Marsamxett Harbour called Fort Tigné. Exactly how Tousard became acquainted with d'Arcon's lunette design is unknown, but the resemblance is too close to be coincidental. It was, like d'Arcon's works, quadrilateral in plan, divided by a traverse with a circular tower keep in the rear and the surrounding ditch was protected by counterscarp galleries. Fort Tigné, however, was a fully defensible and self-contained fort, larger and more sophisticated than d'Arcon's outworks, and is regarded as being the first true polygonal fort.
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