What to do in throne battle?

Yes, you need to attack the trebuchets and the throne itself. Leave your legions inside until your alliance take the throne. It will show up a countdown. The throne can hold up to 5 legions, if i remember correctly, and it's better to have more legions in each trebuchet and the throne to speed up the countdown process. When the countdown it's done, the throne it's yours. After, the R5 of the alliance that took the throne name the Emperor and all the ministers.