Fortress

A fortress is a position that remains drawn even though one side has less material, meaning fewer pieces or pieces of lower value. The defender does not need to recover what was lost. The aim is to build a protected zone that the attacking king and pieces cannot enter and to remove every useful pawn advance, sacrifice, or invasion route.

Signs of a genuine fortress

• The attacking king has no safe entry square.
• The pawns are locked, or any pawn advance creates more risk for the attacker than for the defender.
• A defending piece controls the critical squares and cannot be driven away without allowing another drawing mechanism.
• The defender has safe moves that preserve the setup, so the attacker cannot force zugzwang, a position in which every legal move makes the player to move worse.

A fortress does not end the game automatically. The defender must still answer accurately until the game is drawn by agreement, repetition, stalemate, or another rule. A single capture or king move can open the structure and destroy the defense.

A chess engine, a program that analyzes positions, may report a large advantage while taking a long time to recognize that no progress exists. The practical test is not the evaluation number but the absence of an entry route, a pawn advance or sacrifice that opens the position, or a forced zugzwang.