When the war starts, all applied guilds count as attackers; they treat the defenders and (if not allied) other attackers as foes and may attack them.
Attackers break the Castle Gate, clear the guard towers and reach the Heart; the guild destroying it occupies temporarily. When time runs out, the side holding the heart becomes the new owner.
The official flow chart: the attacking and defending guild's steps through preparation, war start, occupation war and completionThe fortress clerk window: the application period, the war's date and hour, the occupying guild; the application costs 5,000,000 goldThe command post tent: if one has been established, you revive in its vicinity during the warThe phase in which the attacking guild breaks through the castle gate and entersThe fortress foundation: the heart of the fortress, which the attacker destroys and the defender protectsDeath and resurrection rules: you may resurrect 15 seconds after death, and it happens automatically after three minutes