1) Law bonuses increase public order and reduce corruption
2) It can reduce construction times or costs or both.

1) The Romans had a very impressive call-up system and could raise several legions in quick succession. It was common practice in the ancient world that if a battle was lost the victor was the winner, but the romans never gave up. I imagine that the Sack of Rome by the Celts was before the Romans learned this trait.

2) Field Battles were likely lost as soon as one army lost their organisation as war can be damn scary when you don't have your friends egging you on.

3) Depends on the scenario, but mostly once the battle was the lost the survivors would surrender rather than run back to the central plaza and regroup.

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