Originally Posted by Brightgalrs
Oh, I didn't realize it was multiple choice.
I think survival/defense/overwhelmed are essential the same thing, enemy forces assaulting a fortress, and you holding them off.
The difference comes from overwhelmed being using your own city, defense and survival difference in various ways they work, the victory/loss conditions and the way they reward the player and other how they start, survival operates like a trap - you're not expecting to be defending a city, just traveling there for a non-combat assignment, or a much smaller scale one. Small/moderate changes to 'defending a castle' add quite a lot of replay value.