Safe Passage

This is an improvement on the game Chess Contradanse, which I designed and posted here in 1998. It is meant to be a solitaire game that feels as much like standard chess as possible. It could also be played cooperatively with two players.

The goal is have the two kings, and possibly other pieces, switch places without ever putting a non-pawn piece in danger of capture.

Setup

The setup is exactly the same as in standard Chess.

Pieces

The pieces and their movements are exactly the same as in standard Chess, with the exception that there is no en passant capture. There is also no promotion. But castling is still allowed.

Rules

In this solo game, pawns may be put in a position where they can be captured, and may in fact be captured, though there is no obligation to do so. But no other piece may be put into a position where it could be captured at any time. If at any time you notice that a non-pawn piece is in danger, you have lost the game, unless you can immediately backtrack and undo the move that put it in danger.

All rules of standard Chess apply, with the exceptions that there is no en passant capture and no promotion. Castling is allowed. The two colors alternate moves.

There are several levels of goals. The first is to have the two kings change places. Again, no non-pawn piece may ever be put in danger.

A second goal is to have the two queens switch places.

A third goal is to have the other non-pawn pieces change sides, either in the positions of their counterparts or just making it to the far rank. Exchanging pawns may not be possible, since some of them may be captured during play.

I find that by using various openings there is plenty of variety in this game/puzzle. But if you want more variety, you could scramble the opening setup as in Chess 960 or more randomly.

For an easier game, you could allow putting a piece in danger as long as it can immediately move out of danger on the next move. This would be like the difference between check and checkmate.

Notes

I find it helps to use two colored stones or other markers at the side of the board, to keep track of which color piece is moving. I place the white marker in front if it's white turn to move, and the black marker in front if it's black's turn to move. This is especially useful if you walk away from the board for a while and come back to it later.

Inspirations for this variant, besides the original Chess Contradance (and the Seventh Guest bishops puzzle that was an inspiration for it) were the chess-like cooperative/solo games Maze and Diplomatic Mission, by Jim Deacove at Family Pastimes. The Chess Mazes books by Bruce Albertson were also inspirations.