Contravallation is a standard military tactic of siege used in ancient and modern warfare. It is the process of the attacking army building a line of fortifications around the besieged city facing away from the city (to protect themselves from attacks by allies of the city's defenders and to enhance the blockade of the city) and also the resulting fortifications (known as 'lines of contravallation').
Lines of contravallation generally consist of earth ramparts and entrenchments that encircle the besieged city and are built parallel to the lines of circumvallation that face towards the besieged city. This envelopes the city in a double line of fortifications and doubly protects the besiegers. The besiegers may find themselves besieged within their lines of circumvallation and contravallation.
The circumvallation and contravallation tactic has the advantage of making it possible to redirect troops from the siege to the defence of the besieging army itself, with defensive fortifications in either case.