Coral Castle can be found on Home Island. It will unlock at level 9 when you complete the "Investigate the box" quest.
Upgrading Coral Castle will increase the number of slots you have for storage of all items.
It can be upgraded 3 times throughout the game, with each upgrade requiring a different set of items. You can move Coral Castle when you reach level 26, and have upgraded the Castle to level 2.
For more info on upgrading Coral Castle, check the guides we have posted to our official Facebook group and Discord channel.
To learn more about building, check out the Crafting Buildings section!