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I would like to be able to convince myself of the possibility (or lack thereof) of mate using various piece combinations in which the attacker doesn't use their own king.

I'd like to approach this by playing against a strong engine. I'd need to play both the attacking and defending sides at times to get the most out of this as a learning exercise.

If I play the attacking side, I can simply voluntarily not use my king.

If I play the defending side, I can't tell the engine to voluntarily not use it's king. So, for cases where I play the defending side, I need to set up a piece configuration that makes use of the attacker's own king impossible.

So, this post addresses only the case where the engine is attacking and I am defending.

It sounds like some sort of blockade of the attacking king is needed.

The problems I keep hitting to ensure the attacking king can't enter the game are:

  1. Any attacking piece used in the blockade must be immobilized (along with the attacking king)
  2. Any defending piece used in the blockade must be immune from capture
  3. Ideally, any defending piece used in the blockade would also be immobilized. Though I can voluntarily forego moving any defending piece that has available moves, the existence of any such moves might influence the engine's behavior.

So, this has become a learning exercise unto itself. It feels like locked, opposing pawns are key to the solution, but the above problems keep arising. I feel like I am at an impasse.

Does anybody know of a way to do this? Or is it impossible?

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Engines don't usually complain that the initial position you give them isn't legal so any tabiya for the engines immobilized king doesn't have to be legal. Nevertheless I think this tabiya is legal.

[fen "8/1p1p4/b1b5/1p6/1P6/KP6/PP6/8 w - - 1 1"]

There are two ideas:

  1. The white king can't move and hence can play no part unless the structure is broken
  2. The keystone is the black pawn on b5 and the backup behind the pawn is designed to make it as expensive as possible for white to try and break this down.

If the structure is too large, giving too small an area of the board for white to try and mate the black king, or the c6 bishop interferes with the engines plans then the c6 bishop and d7 pawn can be removed, so:

[fen "8/1p6/b7/1p6/1P6/KP6/PP6/8 w - - 1 1"]

If you want a fortress that works for sides, i.e. it doesn't matter if the engine is black or white so that you can play engine against engine then this should do the trick:

[fen "8/pp6/bp6/1p6/1P6/KP6/PP6/8 w - - 1 1"]

The fortress can be made more solid at the expense of becoming illegal so:

[fen "b7/pp6/bp6/1p6/1P6/KP6/PP6/8 w - - 1 1"]
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  • A bit smaller alternative - a position well known to problemists is Kb1 - Pb3 Pb2 Ba1. If you add Ra2 Ra3 Pa4 everything black is well defended. (That said, at OP, White only rarely uses the king in mating attacks. It's more of an endgame thing. Note that the king has a fighting value of 4-5, so White must get a surplus rook or so with regards to classic endgames, e.g. B+N. Also, the fortress in the edge can still produce "false positives", e.g. there is a well known chess problem where White mates with a single Q without K help by driving the black K into a "sinkhole".)
    – Hauke Reddmann
    Apr 22, 2022 at 20:38

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