To begin the mission, go down into the sewers and keep heading towards the marked spot.
That way you will reach an exit leading to the fort.
Optional objective: Reach the signal tower within the given time - 3:00.
Optional objective: Remain undetected on the way to the signal fire.
Optional objective: Limit health loss - 50%.
Optional objective: Completed all the constraints in one playthrough.
After getting outside, silently kill the closest guard and climb onto the building on the right.
Now you can keep jumping between the roofs in the designated order, silently killing the guards on your way. The soldiers' reaction time is delayed, so you should have problems with escaping their sight.
As you reach the last building, jump down to the hay pile below and kill the nearby enemy.
You will now have to wait for the guard on the left to turn around and afterwards quickly run to the right wall of the tower. There you will come across another enemy which you have to eliminate.
With him dead, climb onto the tower and light the signal fire.
After the explosion, head further a bit and you will have to fight with Haytham. To defeat him, you will need to perform counterattacks beside pieces of the environment such as barrels and press the prompted buttons. That way you will end the sequence.