In fortification, formerly, a counterguard.
"The Sailor's Word-Book" by William Henry Smyth
UNLIKE the marches of the land, with guard and counterguard, we had no frontiers on the sea.
"Merchantmen-at-Arms" by David W. Bone
There are also counterguards in front of the salients, whose ends overlap the ends of the tenaille.
"Encyclopaedia Britannica, 11th Edition, Volume 10, Slice 6" by Various