what's the meaning of "Fortress" in chess ?

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Yesandi_Wickramaarachchi
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