A chess fortress is an endgame drawing technique in which the side behind in meterial sets up a zone of protection that the opponent cannot penetrate. ♟️♟️
eg:- W - Kb8 , Bg2 , Nd4
B - Kh8 , Qh7 (black to move)
W B
------ Qh1+
Ka2 Qd5+
Kb8
(=)
BLACK OPPONENT CANT MATE WHITE OPPONENT