Rules of the game Caïssa

Caïssa is a board game much like Chess. It was invented by Christian Freeling. On the website http://www.mindsports.net/CompleteGames/, you will find a collection of his games.

The game is played on a 7 x 7 board with black and white pieces. Both players have a queen, a rook, a bishop and a knight. The diagram shows the board and the pieces in the initial position.

Basic rules

In the initial position, the board is covered by 49 tiles. During the game, the number of tiles will decrease. All play is on the tiles.

White moves first. Players move in turn; it is not allowed to pass on a turn.

The connection rule

The pattern of tiles must always remain connected. That is, a tile or group of tiles must always remain connected (horizontally, vertically or diagonally) to the rest of the tiles. It is not allowed to make a move which would disconnect the tile pattern.

The Queen

The most important piece is the Queen. If your Queen could be captured by the opponent in his next move, you are in check. Whenever you are in check, you must end the check immediately. If you have no possible move which ends the check, the game is over and you have lost. This is called checkmate. It is not allowed to make a move that puts yourself in check.

The diagram on the left shows the Queen's options for movement and capture in a non-check situation: the Queen may move horizontally, vertically or diagonally to all squares with a tile, if the way is free (no other piece is blocking the way). If in check, the Queen is restricted to the adjacent squares, as shown in the diagram on the right. Thus, pieces giving check from a distance need no cover!

The Queen is the only piece with the power of capture in the usual sense: by removing the piece and taking its place. The Queen is not allowed to capture pieces of her own color.

The Atlantis effect

If the Queen moves, the tile she vacates must be removed from the board in the same turn. This process may not violate the connection rule.

The mutual check rule

Queens may not see one another along a horizontal, vertical or diagonal line. It is not allowed to make a move which would cause this situation. Thus, a Queen may protect a piece against capture by its counterpart.

The other Pieces

The pieces move as in Chess:

• The Rook may walk horizontally or vertically to all squares, if the way is free.
• The Bishop may walk diagonally to all squares, if the way is free.
• The Knight moves either one square horizontally followed by two squares vertically, or one square vertically followed by two squares horizontally. A Knight is able to jump over other pieces to reach its destination square.

Under the implicit condition of not putting its own Queen in check, and the explicit condition of not violating the connection rule, a piece can always move to any of its target squares, whether or not it has a tile, and whether or not it is occupied, and if it is, regardless of the color of the occupying piece!

• If the target square has no tile, the piece takes its own tile with it. At the end of the move, the tile pattern must still be connected. There is no 'during the move', so the Knight in the diagram below may legally move to the square marked A. Moving to B is of course illegal.

• If the target square has an empty tile, the piece may simply move there.

• If the target square is occupied by a friendly piece, the two pieces exchange places.
  The Queen is not a normal piece, so it is not allowed to exchange a piece with your own Queen.

• If the target square is occupied by an opponent's piece, the two pieces also exchange places. There is one exception: a switch between two pieces of the same type may not be undone by the opponent immediately in the next move.
  The necessity of the last rule should be obvious. It represents a situation similar to the one that gave birth to the 'ko' rule in the game of Go.

End of the game

For the CodeCup competition, both players can make at most 100 moves. If the game has not ended after both players have made 100 moves, the game ends in a draw.

Example

The mate-in-one below shows another application of the connection rule. For clarity, all other pieces have been omitted. After d2-c4, the Queen cannot move because the tile on c1 would be disconnected. Of course if black were able to exchange the Knight on c4 immediately with any of his pieces, he could yet prevent the mate. But the Queen would remain immobile for the moment.
(The only way to liberate the Queen after an exchange of the Knight on c4, would consist of moving a piece, and thus a tile, to any of the squares b1, c2, d1, or d2, leaving the critical tile on c1 connected if the Queen would move.)