Once you have destroyed the fort’s defences from the sea, it’s time to take over the fort itself. Dock in the area permitted and jump over to the beach.Climb the stairs on the left until you reach a blocked gate. Climb the damaged wall to the left of the gate up to the top. Here, turn to the left and climb up again to reach a larger platform.
Use the scaffolding just to your right to reach the top of this building and kill the officer and any other guards in the immediate area here. Afterwards, head to the far end of this area and enter the door marked with the green objective marker. Kill the enemy inside. This will liberate the fort.