Real Dictionary

contravallation in American English
a fortification set up to protect a besieging force from attack by the defenders of the besieged place or by a relieving force from the outside
noun: a more or less continuous chain of redoubts and breastworks raised by besiegers outside the line of circumvallation of a besieged place to protect the besiegers from attacks from the outside, as by a relieving force

contravallation in British English
noun: fortifications built by besiegers around the place besieged