A bastion is a structure projecting outward from the main enclosure of a fortification, situated in both corners of a straight wall (termed curtain), with the shape of a sharp point, facilitating active defense against assaulting troops. It allows the defenders of the fort to cover adjacent bastions and curtains with defensive fire.
The bastion was designed to offer a full range on which to attack oncoming troops. Previous fortifications were of little use within a certain range. The bastion solved this problem. By using a cannon to cover the curtain side of the wall, the forward cannon could concentrate on oncoming targets.
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 center.
- 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 center. The ground is so low, that if the rampart is taken, no retrenchment can be made in the center, but what will lie under the fire of the besieged.
- A flat bastion is one built in the middle of a courtain, 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 its due proportion of faces, flanks, and gorges.
- A deformed or irregular bastion is that which wants 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.