While normally dormant, a Fortress will occasionally appear with a large skull cloud hovering above it, visible from a distance. This will happen at random after the completion of a previous World Event. When this cloud is present, the Fortress is active with waves of Skeletons manning the island's Cannons.
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 and also cause the other remaining Skeletons to fade away, the Vault Key can be used to unlock a Fortress Vault that contains a large amount of loot.
A skilled sailor could anchor their ship inside the small bay created by the fort's defensive piers and rocks to not get shot at.