This is intended to be a practical and playable 3D chess game. The standard chess pieces are used and their movements are extrapolated into 3D, moving as most players expect them to. The King and Queen may use the 3D diagonal.

I designed this 3D chess game with the following ideas in mind:

•  Preserve the concept of the Pawn being a front line of defense. In 2D, a line of Pawns stands between your pieces and the enemy. In this variant, a wall of Pawns performs this function. Having 36 Pawns and Pieces on each side seemed like too much, so I used the same concept incorporated in my 5x5x5 game: center a smaller army on the rear plane.
• Keep the board isometric. Use a cubic playing field.
• 6x6x6 is the largest field I dare try!
• Use two Kings instead of one and require that they BOTH remain safe. This reduces drawishness and makes the endgame more like orthodox chess, meaning that the advantage of a Pawn or Rook may be enough to win.  A King fork or a skewer is considered mate if BOTH Kings cannot escape check. It appears certain that  KKQ can force mate against KK. 
• Use only standard pieces extended to 3D - no unusual pieces.
• Use full 3D movement. Allow the King and Queen to use the 3D diagonal.

Questions, comments, flames: tolena AT earthlink DOT net.

The Pawn does not have the option of moving two spaces on its first move, there is no en passant capture, and there is no castling. Both Kings must remain safe. Checkmating one King wins the game. If your Kings are forked or skewered and you cannot save them both, you are considered checkmated. Win is by checkmate; stalemate is a draw. Pawns promote by reaching any of the 36 spaces on the enemy's side of the playing field.

In the 'Super Pawns' variant, Pawns may move sideways and capture diagonally sideways in addition to their standard moves. However, the promotion zone is reduced to include only the 16 central spaces where the enemy pieces start.