Source : Webster's Revised Unabridged Dictionary (1913)

Circumvallation \Cir`cum*val*la"tion\, n. (Mil.)
      (a) The act of surrounding with a wall or rampart.
      (b) A line of field works made around a besieged place and
          the besieging army, to protect the camp of the
          besiegers against the attack of an enemy from without.