A bastion is a structure that sticks out from the main wall of a fort, usually shaped like a triangle or square, and is placed at the corners of the fort. A complete bastion has two sides and two ends, and the ends can help protect the main wall and nearby bastions. Compared to the tall, round towers used in the past, bastions provided stronger defense without needing to fight actively and allowed soldiers to defend from a distance using gunpowder weapons. Bastions were part of the most common type of fort design used between the middle of the 1500s and the middle of the 1800s.

Evolution

By the middle of the 15th century, large cannons had become strong enough to make old medieval towers and walls no longer useful. This was shown during the military actions of Charles VII of France, who captured towns and castles held by the English in the later parts of the Hundred Years War. It was also seen when the city of Constantinople fell in 1453 due to the powerful cannons used by the Turkish army.

During the Eighty Years War (1568–1648), Dutch military engineers improved the designs of forts by making the front parts of the structures longer and the walls shorter. This new design was called a bolwerk. To help protect these forts, they added V-shaped structures called ravelins in front of the bastions and walls to shield them from cannon fire.

These ideas were later used in the construction of trace italienne forts by Sébastien Le Prestre de Vauban. These forts were used during the Napoleonic Wars.

Effectiveness

Bastions are different from medieval towers in several ways. Bastions are shorter than towers and are usually the same height as the nearby curtain wall. Towers were tall, which made them hard to climb, but also made them easier for enemy cannons to destroy. A bastion typically had a ditch in front of it. The other side of the ditch was built up above the natural ground and sloped gently downward. This slope, called a glacis, protected most of the bastion from enemy cannons. The distance between the bottom of the ditch and the top of the bastion also made it difficult for attackers to climb.

Unlike late medieval towers, bastions (except for early ones) had flat sides instead of curved ones. This design removed areas where enemies could hide, allowing defenders to shoot at any spot directly in front of the bastion.

Bastions also cover more space than most towers. This allowed more cannons to be placed and gave enough room for soldiers to operate them.

Most surviving bastions are covered with masonry. Unlike tower walls, this masonry was only a retaining wall. Cannonballs were expected to pass through it and be stopped by thick layers of packed earth or rubble behind. The top of the bastion was exposed to enemy fire and usually not covered with masonry. Cannonballs hitting the surface would cause dangerous stone pieces to fly among the defenders.

If attackers captured a bastion, they could use it as a base to launch more attacks. Some bastion designs tried to prevent this. This was done by digging trenches across the back (called the gorge) of the bastion, separating it from the main rampart.

Types

Throughout history, different types of bastions have been used in fortifications:

• Solid bastions are completely filled, with the ground level matching the top of the rampart, leaving no empty space in the center.
• Void or hollow bastions have a rampart or parapet only along their sides and front, leaving a low, open space in the center. If the rampart is captured, the low ground makes it impossible to build new defenses in the center, as they would be exposed to enemy fire.
• Flat bastions are built in the middle of a curtain wall or enclosed court when the court is too large to be protected by bastions at its ends.
• Cut bastions have an indentation, or re-entering angle, at their tip. They were also called "bastions with a tenaille." These were used when the angle of a structure was too sharp. The term "cut bastion" also describes one that is separated from the main area by a ditch.
• Composed bastions have two sides of the interior polygon that are unequal, causing the gorges (the spaces between bastions) to be unequal as well.
• Regular bastions have faces, flanks, and gorges that are all proportionate in size.
• Deformed or irregular bastions lack one of their demi-gorges (half-gorges), making one side of the interior polygon too short.
• Demi-bastions have only one face and one flank. To strengthen a very sharp angle, the tip is cut, and two demi-bastions are placed to form a tenaille, or re-entry angle. They are often used near a hornwork or crownwork.

Gallery

• Castillo de San Marcos (completed in 1695), Florida. A fortress with four bastions.
• Bastion Franz (around 1675), Petersberg Citadel, Erfurt, Germany.
• A bastion of the Copertino Castle (built in 1540), Italy.
• Plan of Geneva and surrounding areas in 1841. Large fortifications with many bastions were among Europe’s most important. These fortifications were demolished ten years later.
• Plan of Tvrđa from 1861, Osijek, Croatia. Most of the fortifications have been removed, but some bastions remain.
• St. Martin’s Demi-Bastion (built in 1614), Cittadella, Gozo, Malta.
• Bastion Middleburg, Malacca, Malaysia.
• A bastion in Badajoz, Spain.
• A bastion located at Nahargarh Fort in Jaipur, India.