Sorry that was clear as mud meant define it precisly plus any other ideas. No real idea when it would occur but my thoughts more happened at specific time frames theatres like city seige otherwise uncommon. If this is the case half of your fortifications could in fact be out of position as in overun at the start of the battle as the lines have shifted.
Possibly a more common occurence is to allow both sides to dig in only as in fox holes. In both cases map generator should probably add a liberal sprinkling of shell holes, never as straight forward as initial thoughts
