Refer to the 1st post in this thread by IvanPixel.
Winter Builder 3 is a community based event in which every player from each region work together to collect "Divine Ice" which they then exchange for "Ice Coins" which can then be spent in the "Shop" to buy camp equipment, precious stones/jewels, green rings & food items.
The "Divine Ice" is found in the 3 story mines called:
- Frozen Cabins
- Frozen Mine
- Frozen Pirate Ship
The "Divine Ice" is also found in the repeatable mines called:
- Icy Cellar
- Icy Cavern
- Icy Island
You can access these mine locations via your "Map" tab.
You exchange the "Divine Ice" for "Ice Coins" via the Winter Builder 3 icon in your camp.
You collect the 8 keys needed to unlock each level of the "Ice Castle In Progress" through the "Achievements" awarded to you for exchanging "Divine Ice" to "Ice Coins".
So you need to gain a minimum of 560 "Ice Coins" to complete all "Achievements" which will reward you with all 8 keys to be able to access each level of the "Ice Castle In Progress".