Polygonal fort

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 proved 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. 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 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.