Object: Checkmate or capture the opponent's Kings.
 
If a player captures a piece, the opponent can move a piece twice, indeed he is forced to do so.
The opponent can forfeit the second partial move only if there is no legal move available anymore for the piece he has moved.
 
Apart from this, the standard chess rules apply.
 
 
More freeware as well as real puzzles and games to be found at http://karl.kiwi.gen.nz/.