Hm, if all the other players should be defeated, use this trigger:
Events
Unit - A unit dies
Conditions
(dying unit) equal to [the castle]
Actions
Game - Victory (Owner of killing unit)
Player Group - Pick every player in (All players matching ((Owner of killing unit) not equal to (Matching player))) and do actions
- Actions
Game - Defeat (Picked player)
didn't test it, but I think it should work