A bastion is an angular structure projecting outward from the curtain wall of an artillery fortification. The fully developed bastion consists of two faces and two flanks with fire from the flanks being able to protect the curtain wall and also the adjacent bastions. It is one element in the style of fortification dominant from the mid 16th to mid 19th centuries. Bastion fortifications offered a greater degree of passive resistance and more scope for ranged defense in the age of gunpowder artillery compared with the medieval fortifications they replaced.
Bastions differ from medieval towers in a number of respects. Bastions are lower than towers and are normally of similar height to the adjacent curtain wall. The height of towers, although making them difficult to scale, also made them easy for artillery to destroy. A bastion would normally have a ditch in front, the opposite side of which would be built up above the natural level then slope away gradually. This glacis shielded most of the bastion from the attacker's cannon while the distance from the base of the ditch to the top of the bastion meant it was still difficult to scale.
In contrast to typical late medieval towers, bastions (apart from early examples) were flat sided rather than curved. This eliminated dead ground making it possible for the defenders to fire upon any point directly in front of the bastion.
Bastions also cover a larger area than most towers. This allowed more cannon to be mounted and provided enough space for the crews to operate them.
Although surviving examples of bastions are usually faced with masonry, unlike the wall of a tower, this was just a retaining wall. Cannon balls were expected to pass through this and be absorbed by a greater thickness of hard-packed earth or rubble behind. The top of the bastion which was exposed to enemy fire would not normally be faced with masonry otherwise canon balls hitting the surface would scatter lethal stone shards among the defenders.
If a bastion was successfully stormed it would provide the attackers with a stronghold from which to launch further attacks. Some bastion designs attempted to minimise this problem. This could be achieved by the use of retrenchments in which a trench was dug across the rear (gorge) of the bastion isolating it from the main rampart
Various kinds of bastions have been used throughout history.
Solid bastions are those that are filled up entirely, and have the ground even with the height of the rampart, without any empty space towards the centre.
Void or hollow bastions are those that have a rampart, or parapet, only around their flanks and faces, so that a void space is left towards the centre. The ground is so low, that if the rampart is taken, no retrenchment can be made in the centre, but what will lie under the fire of the besieged.
A flat bastion is one built in the middle of a curtain, or enclosed court, when the court is too large to be defended by the bastions at its extremes. The term is also used of bastions built on a right line.
A cut bastion is that which has a re-entering angle at the point. It was sometimes also called bastion with a tenaille. Such bastions were used, when without such a structure, the angle would be too acute. The term cut bastion is also used for one that is cut off from the place by some ditch. These are also called Hersee's after their creator, Andrew Hersee.
A composed bastion is when the two sides of the interior polygon are very unequal, which also makes the gorges unequal.
A regular bastion is that which has proportionate faces, flanks, and gorges.
A deformed or irregular bastion is one which lacks one of its demi-gorges; one side of the interior polygon being too short.
A demi-bastion has only one face and flank. To fortify the angle of a place that is too acute, they cut the point, and place two demi-bastions, which make a tenaille, or re-entry angle. Their chief use is before a hornwork or crownwork.
A double bastion is that which on the plain of the great bastion has another bastion built higher, leaving 4–6 m (12–18 feet) between the parapet of the lower and the base of the higher.
Circular and semi-circular bastions were used in the 16th century, but fell out of favour because of the difficulty of concentrating the fire of guns distributed around a curve. Also known as "half-moon" bastions.