Towards the end of the 18th century, when the Vauban style of fortification was no longer wanted or able to be used (the cost of this type of fortification knew no bounds, because to reinforce it another ring of bastions had to be built around the existing one(s)), military construction engineers began instead to build self-contained outworks in front of fortresses (e.g. the polygonal system). These outworks were as a rule in the shape of a bow facing the likely enemy approach and designed primarily to defend attacks from that direction. The "chord" of the bow was only weakly fortified (the attacking artillery could not normally use it - an exception is e.g. Fort Douaumont, when after its capture by the Germans, the gorge was suddenly used by the enemy, i.e. the French) and hence the most vulnerable side of an outwork or fieldwork – hence the name "gorge". The design of an outwork or fieldwork was such that its gorge could still be reached from the fortress or main defensive position by artillery or rifle fire and could therefore still be covered by fire.
- Hartwig Neumann: Festungsbaukunst und Festungsbautechnik. Deutsche Wehrbauarchitektur vom XV. – XX. Jahrhundert. Mit einer Bibliographie deutschsprachiger Publikationen über Festungsforschung und Festungsnutzung 1945–1987. 2nd edition, special edition. Bernard & Graefe, Bonn, 1994, ISBN 3-7637-5929-8, (Architectura militaris 1).