The besieging of a position by encircling it with ditches and ramparts.
siege lines built to "invest," or enclose, the enemy's defenses. See Countervallation.
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'.