The help file for Castle Vox is vague:
Conflicts are resolved by the game according to the attack/defend values of the units involved.
And here's all it says about the attack/defend values:
The pawn has a strong defensive value, but a weak attacking value.
knight has a weak defensive value, but a strong attacking value.
Castles provides an extra defensive bonus.
It doesn't give specific numbers , if there are any other factors (such as a random die roll), or what formula it uses to crunch the numbers.