Demi-bastion

A bastion is an angular structure projecting outward from the curtain wall of a fortification, most commonly at the corners. 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.

By the middle of the 15th century, artillery pieces had become powerful enough to make the traditional medieval round tower and curtain wall obsolete. The was exemplified by the campaigns of Charles VII of France who reduced the towns and castles held by the English during the latter stages of the Hundred Years War, and by the fall of Constantinople in 1453 to the large canon of the Turkish army.

To counter this weakness, during the 16th and 17th centuries, the height of the walls was reduced and thickened and the bulwarks were moved close enough to the fortifications that their rear sides could be protected by curtain walls, so forming bastions. Initially these bastions were fairly small circular (see for example the fortress build on Rhodes between 1486 and 1497 and the semi-circular bastions at Deal Castle (built 1539–1540).

During the Eighty Years War (1568–1648) Dutch military engineers developed the concepts further lengthening the faces and shortening the curtain walls of the bastions. To augment this change they placed v shaped outworks (Ravelins) in front of the bastions and curtain walls to protect them from direct artillery fire.

These ideas along with others would be incorporated into the trace italienne forts by Sébastien Le Prestre de Vauban, in which were many bastions of a type that would still be in use during the Napoleonic Wars.

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Wikipedia