OK, the engine's now at about 75%. The opponent AI isn't quite right, and needs a little extra tweaking, but for the most part the actual logic of the game is in place.

-=-=-

Now comes the hard part.
I need to decide exactly what sort of game this will be.
One long endless game, or lots of little puzzles to contend with.
.. Or maybe both, with an option between the two modes?
Hmm..

I expect a lot of today will be about tackling the AI/Level Generation to ensure that short puzzles are always beatable, and from there making the final decision as to what form the game will take.