From what I could gather, the defenders objective is to simply stop the attackers from advancing. They can do this in the 3 seperate stages by killing the attackers, who have a limited number of tickets to respawn with, so reviving your team will be essential. The defenders get ballista, boiling oil and other defences to stop the attackers, who push a battering ram to the gates of the castle, and then through into the next stage, where they have to break through to the inner courtyard. The final stage had a lord, or a king, that is an AI with a large amount of health that the attackers have to kill to win. The attackers start with a battering ram, I think they have to stay close to it like with the campaign, and like the campaign if the defenders get close the battering rams progress slows. Once that reaches the main gate it breaks it down, then pushes through. In the second stage, the defenders have 3 points that spawn archers, and other defences. If the attackers capture one of those objectives, it’s permanently theirs.
I think that’s the gist of it. May have missed something though