Karstedt's fortress

Karstedt's fortress is a defensive position in queen versus bishop and knight. A fortress is a formation that cannot be broken despite a large material disadvantage, provided the defender keeps the pieces coordinated.

In the Karstedt setup, the king, bishop, and knight form a barrier of controlled squares. The queen can check and switch sides quickly, but it cannot force the defending king to abandon the formation or create a useful route for its own king to enter.

This creates an important exception: a queen normally wins against bishop and knight, but not when the defender reaches this fortress. The practical question is not just how much material each side has, but whether the defender can complete the setup before the queen separates the pieces or brings its king close.

The fortress belongs to pawnless ending theory. It is not any compact cluster of pieces. One wrong square, an unprotected piece, or a displaced king may let the queen enter with checks, double attacks, or an attack that immobilizes a piece because moving it would expose the king.