Circumvallation is a standard military tactic of siege used in ancient and modern warfare. It describes the process of the attacking army building a line of fortifications around the besieged city facing towards the city (to protect itself from sorties by its defenders and to enhance the blockade). The resulting fortifications are known as 'lines of circumvallation'.
Lines of circumvallation generally consist of earth ramparts and entrenchments that encircle the besieged city. The line of circumvallation can be used as a base for launching assaults against the besieged city or for constructing further earthworks nearer the city.
In cases where the besieging army is threatened by a field army allied to the city, the besieging army may construct a second line of fortifications between itself and the outside enemy parallel to the lines of circumvallation, known as 'lines of contravallation'. This envelopes the city in a double line of fortifications and doubly protects the besiegers. In turn, the besieging army may find itself besieged within its lines of circumvallation and contravallation.
The circumvallation and contravallation tactic has the advantage of making it possible to redirect troops from the siege (now reinforced by the first wall of fortifications) to the defence of the besieging army itself.
Examples of circumvallation in military history