circumvallation

noun

1. A rampart or other defensive entrenchment that entirely encircles the position being defended.

   “[...] and in a word, would intrench and fortify them round with as many circumvallations and breast-works, as my uncle Toby would a citadel.”

   “The Saxon architect had exhausted his art in rendering the main keep defensible, and there was no other circumvallation than a rude barrier of palisades.”

   “He just managed to finish her in time—the day before the date fixed for his breaking ground on a greater business still, the circumvallation of Mrs. Dunn.”