Read the wiki: Sunfire Castle - Whiteout Survival WikiÂ
Quick summary:
The "winner" is either the alliance who holds Sunfire castle for 3 consecutive hours in the 6-hour event, or whichever alliance has held it the most cumulatively at the end.
The longer you hold a turret, the stronger it becomes, killing more defenders inside the castle
Everyone inside the Alert zone will be teleported randomly 2 hours before the event. You can join the battle by teleporting to the Alert Zone or attacking from outside the alert zone from on hour before the event until it ends
If you teleport to the Alert Zone, you will deactivate any active shield and it cannot be reactivated
If holding the castle, you need a lot of infantry: the turrets will whittle them down and if you don't have infantry left the castle will be lost to rallies
Obtaining Points:
Carnage points: defeat enemies in the Sunfire Castle and Turrets
Occupation points: gained by troop power occupying the Sunfire Castle or Turrets
Casualty points: awarded based on Severely Injured or Lost troops
This is a good way to gain points: solo attack, take casualties, and batch heal in small increments. Tap Tap heal to help your alliance, but have lots of resources banked to support this as well!