I was looking for something on what the Romans who were surrounded at Cannae experienced and did, as the outer ranks were gradually destroyed, up until the end of the battle.

Any other pre-gunpowder battle where an army was trapped in a pocket, actually. I am curious if they built up impromptu HQs, collected wounded, planned breakthroughs, or generally acted in a manner similar to the massive encirclements on the WW2 eastern front.