Circumvallations

Circumvallations are lines of fortifications constructed by a besieging army around a fortress or besieged town in order to isolate it, prevent relief forces from approaching, and protect the siege works and attackers’ supply lines. A circumvallation typically forms a continuous belt of ramparts, trenches, redoubts, and other fortifications connected by covered ways and artillery positions. The goal is to force the defender to surrender through attrition and to prevent sorties and external assistance.

Contravallations are outer lines built outside the circumvallation to defend the besieging army from relief forces attempting to break the siege. In practice, a siege often involved both circumvallations and contravallations: the inner line isolates and engages the town, while the outer line shields the besiegers from outside attack and secures their own lines of communication.

Construction and layout varied with terrain and period, but common elements included glacis and trenches, outworks such as lunettes or redoubts, covered passages, and artillery emplacements. The lines could encompass the entire circumference or be concentrated where relief threats or terrain dictated.

Historical use of circumvallations was prominent in early modern warfare, especially during the Italian Wars and the Dutch Revolt, when gunpowder artillery made direct assaults costly and prolonged sieges more common. With changes in artillery ranges and logistics, the formal use of extensive circumvallations declined in later centuries, though the strategic idea of surrounding and isolating a fortified place persisted in various siege tactics.