Definition of "circumvallation"

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

1761, Laurence Sterne, The Life & Opinions of Tristram Shandy, Gentleman, volume 3, Penguin, published 2003, page 201

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.

1819, Walter Scott, Ivanhoe

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.

1901, Henry James, Flickerbridge