[Kobal2]

All other things being equal, attack B. There are three things that can happen:

- the battle happens in B. This leaves the fort in A open for recruitment, reinforcement, and an exit route if you lose the battle
- the battle happens in A. This is functionally not different from choosing to patrol A.
- (rarely) both armies skip by each other. The AI is sieging A, but is now cut off because you took B. Doubling back will ensure they die to the last man (as long as you can otherwise hold B, which shouldn't be too hard against the AI).

Putting ~10 points more PD in C than in A should ensure the AI never sets foot in there.

Against humans... too many variables, really. Not to mention that the "penalty" for attacking, that is to say the one buffing turn the defender gets, is much more important when it's a human milking it for all it's worth.