fortification built around besieged place
• (n.) A trench guarded with a parapet, constructed by besiegers, to secure themselves and check sallies of the besieged.
Con`tra·val·la'tion noun [ Prefix contra- + vallation : confer French contrevallation . Confer Countervallation .] (Fort.) A trench guarded with a parapet, constructed by besiegers, to secure themselves and check sallies of the besieged. 
In fortifications a contravallation was a trench guarded with a parapet, constructed by besiegers, to secure themselves and check sallies of the besieged.
contravallation 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.
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.