What is "countermure"

Countermure \Coun"ter*mure`\ (-m?r`), n. [Counter- + mure: cf. F. contremur.] (Fort.) A wall raised behind another, to supply its place when breached or destroyed. [R.] Cf. Contramure.
--Knolles.

Countermure \Coun`ter*mure"\ (koun`t?r-m?r"), v. t. [imp. & p. p. Countermured (-m?rd"); p. pr. & vb. n. Countermuring.] To fortify with a wall behind another wall. [R.]
--Kyd.

n. A wall raised behind another, to supply its place when that is breached or destroyed. vb. To fortify with a wall behind another wall.