The board is a cube, five spaces on a side. There is no pawn double-step, no en passant, and no castling. Win is by checkmate; stalemate is a draw.
 
I designed this 3D chess game with the following ideas in mind:

1. 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. I didn't like set-ups in which the armies begin diagonally opposed (for example, Raumschach).
2. Keep the board isometric. Use a cubic playing field.
3. 5x5x5 seems to be the best size for a cubic field.
4. Try to use the standard set of pieces.
5. 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. I believe that KKQ can force mate against KK.
6. Use only standard pieces extended to 3D - no unusual pieces.
7. Use full 3D movement. Allow the King and Queen to use the 3D diagonal.