The Collaborative International Dictionary of English v.0.48:

Countermure \Coun`ter*mure"\ (koun`t?r-m?r"), v. t. [imp. & p.
   p. Countermured (-m?rd"); p. pr. & vb. n. Countermuring.]
   [Cf. F. contremurer.]
   To fortify with a wall behind another wall. [R.] --Kyd.
   [1913 Webster]