Fortresses are very distinct from other islands by the wooden structures that surround the waters. Cannon Towers, and more notably the keep, provide long-range defense against incoming vessels while active.
While inactive Fortresses can also serve as resource dumps where the many barrels of supplies can be looted for your ship including Bananas, Wooden Planks and Cannonballs. A vast number of Gunpowder Barrels are also stored amongst the structures, where they can be either use in combat or taken for profit.
When a Skull Cloud appears over one, that Fortress is active and can be raided, more information can be found here.
List of Fortresses
|Keel Haul Fort||C-7||The Shores of Plenty|
|Hidden Spring Keep||K-9||The Shores of Plenty|
|Sailor's Knot Stronghold||F-17||The Shores of Plenty|
|Kraken Watchtower||O-6||The Wilds|
|Shark Fin Camp||U-5||The Wilds|
|Skull Keep||U-11||The Wilds|
|Old Boot Fort||P-17||The Ancient Isles|
|Lost Gold Fort||J-21||The Ancient Isles|
|The Crow's Nest Fortress||S-22||The Ancient Isles|