In the rules, it says strongholds are attacked just like cities. I does not say that they are captured like cities. In the rules it says that the battle ends immediatly if the stronghold's strength is reduced to zero, but does not say anything about victory points.
If I assume that strongholds are taken like cities, I would assume that when all nobles retreat, you have the option of rasing cities, so it would make sense that you could raise a stronghold with a partial victory.
So do you have to reduce a stronghold to zero to destroy it, or can you also win with a total or partial victory?