A Fortress is a type of Island that hosts Fortress Raids. A fortress is defined as any location that is marked and named, that does not spawn Animals and cannot be subject to Quests, and is not a Outpost.
Fortress Raids
An ominous skull cloud appears over active Fortresses waiting to be raided.
While normally dormant, a Fortress will occasionally appear with a large skull cloud hovering above it, visible from a distance. This will happen approximately three hours after the last raid on the same server has been completed.
When this cloud is present, the fortress is active, meaning that a Skeleton horde has gathered there, and pirates can choose to engage in a raid of that Fortress.
Upon arriving at an active Fortress, pirates can fight their way through waves of Skeletons to the central Fortress structure. Horns signal the arrival of new waves of Skeletons, which occurs briefly after all Skeletons in the previous wave are defeated. There are roughly 10-15 waves per raid.
At the final wave, the Fort's Skeleton Captain will spawn. It will drop a Vault Key upon death, which can be used to unlock a Fortress Vault that contains a large amount of loot. One crew will generally need to make multiple trips to collect it all.
Raiding a Fortress can prove highly difficult, and multiple pirate crews may wish to team up in order to tackle it.
The skull cloud can only appear at one of these three Fortresses: The Crow's Nest Fortress, Shark Fin Camp and Keel Haul Fort. Every other Fortress is permanently inactive at the moment.
Fortresses contain large amounts of Cannonballs, Planks, Bananas and Gunpowder Barrels at all times, even when no raid is active there.
The eyes of the skull cloud are not an indicator for the difficulty or other ships near the fort.
Fortress Vault
The Vault is unlocked using the Vault Key dropped by the Skeleton Captain. It contains a large amount of randomized Treasure, worth between 8000 and 20,000 Gold in total. The Stronghold Skull and Stronghold Chest are guaranteed to spawn in the center of the Vault, on a raised alter. Gunpowder often spawns in the Vault, but is typically detonated during the fighting before the Vault is opened. There is no way to open the Vault without the Vault Key. While the Vault Key can be used to open any Vault, only the Vault of the Fortress that was recently raided will contain any Treasure.