A reasonable system, if I may say so. Would possibly benefit from a little tweaking, however. Here are my suggestions:
1. Change the starting setup to
d d d d d d d d
a a a r a a a
a fifteen piece setup fits a triangular board better, with symmetry. Remember though that even with a reduction like this, the board will be huge. One advantage is that with the board as it is you can play with three players - possibly a more interesting game. The other option is to reduce the number of pieces and make the board smaller.
2. I like your differentiation between the movement styles for the pieces. Very elegant. You must, however, clarify the defender's method of capturing - this would potentially allow two different moves to be made to capture the same piece, with different end points (though this could be a good thing, I suggest keeping it, gives them more flexibility). Also, can a defender capture if it'll end up in an occupied square? Silly question, but it needs to be asked. 'Backwards' also needs to be defined in relation as well - I suggest from the mid-points of the triangles in question.
3. I take it that 'any direction' is intended to mean in /\/\/\/\/\/ rows. As opposed to
any direction, which could be interesting.
Just some constructive criticism, I think this has great potential, and I mean it. Have you playtested to check for noticable first/second mover advantages? I suggest also playtesting the three player version.
Great game! Just wish I had thought of it first, my agent loves abstract games.
Edit: Modified board
