A fortress is a defensive setup so solid that the stronger side can’t break in — the position is drawn even though one player is down material.
Normally an extra piece or pawn wins. A fortress is the exception: the defender builds a wall the enemy simply can’t get past, so the extra material is useless and the game is a draw no matter how long it goes on.
The most famous is the ‘wrong bishop’ rook-pawn draw: a king reaching the corner that the bishop can’t cover stays put forever, so bishop plus rook pawn fails to win. Others include blockades with opposite-coloured bishops and a king walling off a passed pawn.
Spotting a fortress saves lost games and warns you off ‘winning’ material that leads nowhere — sometimes a draw is the best the extra pawn can give.
A bishop whose colour doesn’t match the promotion square of its rook pawn. The defending king reaches that corner and can never be driven out, so the extra bishop and pawn only draw.
A fortress is a positional wall the defender holds indefinitely while down material — there’s no breakthrough at all, as opposed to draws that come from perpetual check, stalemate tricks, or simply equal material.
BetterChess is a practice tool — we make no guarantee you'll reach 1800 or any rating. Definitions are standard chess terminology; every diagram position is checked legal with the same engine the board runs.