The bulwark or bastion is a fortified redoubt that projects outward from the main body of a fortress "Fortress (architecture)"), generally located at the corners of the curtain walls "Curtain (architecture)"), as a strong point of defense against the assault of enemy troops.

It can have a pentagonal shape and, in this case, it is made up of two faces, which form the protruding angle; two flanks, which join it to the curtain; and the goal line through which you enter the bastion. The design of the bastion and the fact that it protrudes from the body of the fortress allows the other bastions and curtain walls to be covered by crossfire.

The angle formed by the two faces is called the bastion angle; The one that forms the face with the flank is called the back of the bastion; and the union of the flank with the curtain, angle of the flank. The gola is usually broken, composed of two straight lines that continue the direction of the curtain walls it defends. The imaginary line that joins the vertices of the interior angle and the exterior angle is called the capital: its measurement gives the extension of the bastion.[2]​.

The bastion was used as an artillery platform, which forced the assailant to place his artillery batteries further away from the walls, thus decreasing their effectiveness.

The bastion was developed at the end of the century in Italy as part of the so-called Italian trace and expanded throughout Europe at the beginning of the century. Its adoption is a consequence of the inefficiency of traditional fortifications (high, thin walls) in front of the canyon.[3]​.