A bastion is a structure projecting outward from the curtain wall of a fortification, most commonly angular in shape and positioned at the corners of the fort. It is a fully developed structure that differs from medieval towers in several ways. 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. There are different types of bastions, including flat bastions, cut bastions, and more.
In addition to its military meaning, the term "bastion" can also be used metaphorically to refer to anything seen as preserving or protecting some quality, condition, etc. For example, British public schools are regarded as one of the last bastions of upper-class.
Furthermore, in the field of computer security, a bastion host is a specialized computer that is deliberately exposed on a public network. From a secured network perspective, it is the only device directly accessible from the outside world.
