A fortress is a defensive technique in which the materially inferior side builds an impregnable position, forcing the opponent to accept a draw because they cannot break through.
The idea relies on creating a fortified zone around the king or controlling key squares that prevent any further advance. For instance, in certain rook-versus-queen endgames, the weaker side can tuck their king into a corner and maintain a configuration where no decisive progress is possible — the opponent may have a theoretical advantage, but cannot convert it without a defensive mistake.
In practice, recognising a fortress is just as important as building one. When facing a significant material deficit — a piece or several pawns down — check whether your king can shelter in a zone the opponent cannot penetrate, and whether your remaining pieces are enough to deny entry. A fortress does not happen by chance: it requires identifying the critical squares to control and never letting your guard down.