Again, the defences on this fort are quite laughable and a few decent mortar shots mixed in with a broadside/heavy shot barrage or two will knock most of them over in no time. When the defences have been dealt with, we want to sail over and dock in the indicated green beam of light.
Hop out here and follow the path. Climb the stairs until you reach the blocked gate. Climb this up to the top and drop down the other side. Turn to the right and make tracks for the building ahead with the red objective marker killing any enemies that try to block your path.
When you reach the building use the wall run to climb up the door and grab the wooden beam above. Enter the window here. Turn right and clear out the balcony through the next doorway. Climb up the cages stacked against the wall and use them to access the higher platform. Once up, climb up to the roof.
From the roof, move over to the paved area with the garden nearby and move to the edge where the red objective marker is. Highlight the target and hit the assassinate button to perform an air assassination. Afterwards, climb back up and enter the door marked with the green objective marker. Kill the enemy inside. This will liberate the fort.