Here is an idea...
Every inactive Fort has NPC pirates that man the cannons and provide refuge against nearby skeleton ships.
Skeletons ships may sometimes spawn around an inactive fort and begin attacking it. A new cloud in the sky signals a Fort defense event. Skeleton ships take no damage from the Fort whilst this event is up and active (so they don't get sunk before you arrive)
Sink the ships then assist the NPC pirates defend the Fort killing the remaining skellies
On the land Skeletons and NPC pirates fight it out.
Kill the captains and the boss to end the seige and get a small bit of loot from the treasury.
If the seige is not assisted by you eventually the inactive Fort will become an active one.
Occasionally forts become active without seige just so we don't get it where you never see active forts again.
Kill the ships before you kill the skellies on land as the ships are responsible for spawning the skellies, spawning with green visual magic effect.
But it will give the illusion skelly ships are trying to take territory and human sailors and pushing back etc.
