thanks HappyDog! I Was put a little bit under pressure (in a positive way) by Fl0rrat to develop this type of puzzle. the first version was a simple 'chessboard' type puzzle but without any interaction between consecutive rows and colums. I like this version much more.
I agree - A strict chessboard appears interesting, but as far as I can see it would just be two separate puzzles on the same grid. Having some interplay is much better.