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Bastion

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A bastion is a structural element that projects outward from a fortification's curtain wall.

Bastion (PSF)
Bastion (PSF)

Most bastions feature an angular shape and are positioned at the corners of a fort. A fully developed bastion consists of two faces and two flanks. The flanks are essential because they allow fire to protect the curtain wall. They also protect the adjacent bastions in the defensive line. This architectural style became the dominant method of fortification from the mid-16th to the mid-19th centuries. It represented a major shift in how military engineers designed defenses against gunpowder artillery.

To understand how a bastion works, one must look at its specific geometry and construction. Unlike medieval towers, bastions are typically lower and similar in height to the curtain wall. High towers were easy targets for heavy artillery to destroy. A bastion usually features a ditch in front of it. On the opposite side of this ditch, the ground is built up and then slopes away gradually. This slope is called a glacis. The glacis serves to shield the bastion from an attacker's cannon fire. Meanwhile, the distance from the ditch base to the top makes the wall difficult to scale.

The shape of the bastion also provides a tactical advantage through improved lines of sight. Medieval towers were often curved, which created "dead ground" where attackers could hide.

Komárno - Castle
Komárno - Castle

Bastions are typically flat-sided rather than curved to eliminate this problem. This allows defenders to fire upon any point directly in front of the structure. Furthermore, bastions cover a larger area than most traditional towers. This increased surface area allows more cannons to be mounted. It also provides enough space for the crews to operate those cannons effectively.

Construction materials were chosen carefully to handle the impact of cannonballs. Most surviving bastions are faced with masonry, which is stone or brickwork. However, this masonry acts only as a retaining wall. It is not meant to stop a projectile by itself. Behind the stone, there is a great thickness of hard-packed earth or rubble. When a cannonball hits the masonry, it passes through and is absorbed by the earth.

Rhodos185
Rhodos185

Engineers often left the top of the bastion exposed rather than using masonry. This was done because hitting stone surfaces could scatter lethal shards among the defenders.

The evolution of these structures was driven by the increasing power of artillery. By the mid-15th century, cannons could make traditional round towers and curtain walls obsolete. This was seen during the campaigns of Charles VII of France. It was also evident during the fall of Constantinople in 1453.

Deal Castle - geograph.org.uk - 1406977
Deal Castle - geograph.org.uk - 1406977

During the Eighty Years War (1568–1648), Dutch engineers further developed these concepts. They created the bolwerk by lengthening the faces and shortening the curtain walls. They also added V-shaped outworks called ravelins to protect the walls from direct fire. Later, Sébastien Le Prestre de Vauban incorporated these ideas into the trace italienne style of forts.

Engineers designed several different types of bastions to fit various terrains and needs. A solid bastion is filled entirely with earth so the ground is even with the rampart. In contrast, a void or hollow bastion has a rampart only around its faces and flanks. This leaves an empty space in the center. A flat bastion is built in the middle of a curtain or enclosed court.

Aerial image of Neuf-Brisach (view from the southwest)
Aerial image of Neuf-Brisach (view from the southwest)

If a court is too large, a flat bastion helps defend the center. A cut bastion features a re-entering angle at its point, sometimes called a bastion with a tenaille.

Other variations include the demi-bastion, which has only one face and one flank. Bastions can also be classified by their symmetry. A regular bastion has proportionate faces, flanks, and gorges. An irregular or deformed bastion lacks one of its demi-gorges. Some bastions are composed, meaning the two sides of the interior polygon are unequal. To prevent attackers from using a captured bastion against the fort, engineers used retrenchments. A retrenchment is a trench dug across the rear, or gorge, of the bastion. This isolates the structure from the main rampart and provides a secondary line of defense.

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🖼️ Images & Media (5)
File:Bastion (PSF).jpg
Bastion (PSF).jpg
File:Deal Castle - geograph.org.uk - 1406977.jpg
Deal Castle - geograph.org.uk - 1406977.jpg
File:Aerial image of Neuf-Brisach (view from the southwest).jpg
Aerial image of Neuf-Brisach (view from...
File:Komárno - Castle.jpg
Komárno - Castle.jpg
File:Rhodos185.JPG
Rhodos185.JPG
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