Investment (military)

Investment is the military process of surrounding an enemy fort (or town) with armed forces to prevent entry or escape. It serves both to cut communications with the outside world and to prevent supplies and reinforcements from being introduced.

A contravallation is a line of fortifications, built by the attackers around the besieged fortification facing towards an enemy fort to protect the besiegers from sorties by its defenders and to enhance the blockade. The contravallation can be used as a base to launch assaults against the besieged city or to construct further earthworks nearer to the city.

A circumvallation may be constructed if the besieging army is threatened by a field army allied to an enemy fort. It is a second line of fortifications outside the contravallation that faces away from an enemy fort. The circumvallation protects the besiegers from attacks by allies of the city's defenders and enhances the blockade of an enemy fort by making it more difficult to smuggle in supplies.

Lines of contravallation and circumvallation generally consist of earthen ramparts and entrenchments that encircle the besieged city.