Fortress
A fortress is a position the weaker side can hold indefinitely: its king, pieces and pawns cover every entry route, so the side with extra material has nothing to aim at and the game ends in a draw.
Extra material wins only if it can be turned into something: a new queen, a mate or a won pawn ending. A fortress is any arrangement that blocks all of those routes at once. The defender's moves do not need to be clever; they only need to keep the arrangement intact.
Two questions tell you whether you face one. Can the stronger king find a way in, through a square or a file the defender does not cover? Can the stronger side force zugzwang, so that the defender has to dismantle the setup? If the defender always has a harmless move and every entry is covered, the position holds.
Corners are the most common home. Against rook pawns a king tucked into the corner is very hard to evict, as in the diagram, and adding a bishop that cannot control the corner square changes nothing. Blockades with opposite-colored bishops and closed pawn chains where the extra piece has no target are other frequent forms.
When you are behind, look for one before the position simplifies further, and trade toward the pieces that cannot break it. When you are ahead, keep a second way to play, such as pawns on both wings, so that the defender never gets to settle into a single fortress.
A neighboring idea is Rook versus minor piece. Rook versus minor piece is an ending or imbalance in which one side has a rook and the other a bishop or knight, so the result depends on the pawns and on how active the pieces are.
Also worth reading is Wrong-colored bishop. A wrong-colored bishop runs on the opposite color to the queening square of its side's rook pawn, so it can never cover that corner or drive out a defending king sitting there, and the ending is drawn.
Also worth reading is Stalemate trick. A stalemate trick is a resource for the side that is losing: it forces the stronger side into a position where the weaker side has no legal move and is not in check, so the game is drawn.
If "Fortress" has cost you half a point, a first lesson is a good place to find the pattern behind it: George Li goes through your games and builds a plan.
Related terms
A theoretical draw is a position in which perfect play by both sides leads to a draw, so neither player can force a win no matter how many moves are left.
A wrong rook pawn is an a-pawn or h-pawn whose promotion square is the opposite color to its side's bishop, so king, bishop and pawn cannot win once the defending king reaches the corner in front of it.
A wrong-colored bishop runs on the opposite color to the queening square of its side's rook pawn, so it can never cover that corner or drive out a defending king sitting there, and the ending is drawn.
Stalemate is a draw that occurs when the side on move has no legal move at all while its king is not attacked, so the game ends on the spot with half a point to each player.
A material advantage means one side has more total piece value on the board than the other, for example an extra pawn or an extra piece, and can usually convert it by trading down.
Opposite-colored bishops are bishops that run on different shades of squares, so neither side can attack or exchange the other's bishop, which makes endings with them notoriously drawish even when one side is a pawn or two ahead.
Guides that go further
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
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A game review, a read on your level and a written study plan from George Li.
Want to spot this in your own games?
A first lesson starts with your games: George reviews them and builds a study plan around what he finds.