Part 1: Naval Combat
Port la Joye is relatively easy as far as forts go as long as you can account for the conditions. The fort is located in a narrow channel and is often guarded by a brig or two. It has the usual mortar fire and four cannon towers, but should be easy enough to take down as long as you draw the hostile ships away from the fort and sink them so that you can attack it in peace. Once you have cleared out the defences using ‘ye good olde’ mortar/broadside combination, dock at the area indicated.
Part 2: Infiltrate
Upon docking, duck through the hole in the wall to the right of the gate and use the stalking zone to look around the corner for the first powder reserve. Shoot this to destroy it and kill the guards nearby. Assassinate any remaining enemies here and then climb the tower opposite the hole we entered and kill the first sniper at the top.
From the tower here, use the rope to access a rooftop to the north and continue across the next rooftop and up the rope to reach the second sniper in a guard tower. Take him down as well. Take his weapon and using the rifle, you should be able to shoot the second powder reserve in the open area in front of the large building ahead. Destroying this will bring out the commander, so approach and kill him to complete the fort liberation.