In order to attack a fort, sail up into its region. Note that the fortifications are in the restricted areas. There is a chance that the enemy ships join in also. The forts cannot be taken by surprise and that is why the barrage starts after you sail up to it.
Destroying of fortifications
There are three means of defense to each fort. The towers shoot cannon salvos at large distances. The walls shoot heavy shots salvos at short distances. Mortars attack over the longest distance and they are the only ones that cannot be damaged. Your aim, at this point, is to destroy all of the designated towers and walls.
The best tactics to adopt is to avoid the mortar fire and discharging your own mortar. Destroy the leftmost, or the rightmost tower and sail up to that spot. Once there, you can sail ahead, slowly, and destroy the successive fortifications, carefully, or sail ahead fast, avoid the cannon and mortar fire, while taking frequent U-turns. Be careful at the high-difficulty forts, because one salvo is capable of inflicting heavy damage to the Jackdaw's hull.
Accessing the fort
After you have destroyed the fortifications, the coast will be highlighted. Moor there and go up. The gates will always be locked so, you need to climb up. Depending on the kind of the fort, on the left, or on the right, there is a wall that you can climb onto.
Kill the officers
Inside, there is a fight under way, between the pirates and the fort's crew. Your aim is to kill one to three officers, depending on the individual fort. The crew does not realize your presence, so you can sneak past them to reach your targets and eliminate them quietly. You can also attack them in an open fight but, with larger numbers of soldiers, this may be problematic. If necessary, use your allies support.
Kill the commander
After you have killed the officers, you need to reach the conference room. After you force the door open, you only need to assassinate the fort's commander, and Edward will claim the fort for himself.
A seized fort works in the very same way as beauty spots. It reveals the hidden collectibles and places on the map of the Caribbean. Additionally, each fort becomes a field base for the Jackdaw. In there, Edward will be able to upgrade or repair the ship, as well as to take up the naval missions.