CIRCUMVALLATION, LINES OF, in fortification, a continuous circle of entrenchments surrounding a besieged place, facing outwards, by which the besieger protected himself against the attack of a relieving army from any quarter (from Lat. circum, round, and vallum, a rampart). "Lines of contravalla tion" were similar works but naturally of less circumference, facing inwards, as a resistance against the sorties of the besieged.