The Hidden Blade Assassin's Creed in Depth

Speedrun
The fastest way I was able to liberate the fort. Luckily there is no restricted area in the place, which makes stealth a whole lot easier. Start off by using the fast travel station in the northeast part of New York (sure beats having to run all the way there). After that move along the wall until you reach the low hanging beams. You can reach the one on the right by either running up the wall under it or running up the wall beside it and kicking to the side. Keep moving forward along the wall and turn left. Run straight ahead to destroy the powder reserve. Head back the way you came. Once you are back on the beam, run up the wall and kick to the left to reach the cannon above. From here you can shoot the fort's captain, who is patrolling below on his horse (shooting with a bow is a little slower, but doesn't attract as much attention). As it just so happens, the flag you need to lower is right in that area. I could have done this several seconds faster, but it didn't need to be perfect.

Air Assassination and Stunt
The highest spot in the fort is the top of the flag pole. Yes, you can climb up it. In the captain's travel route there is a spot where he can be air assassinated. Lock onto him to avoid accidentally air assassinating one of the nearby guards. The cool thing about this is that by the time you've killed him his horse turns the corner, keeping you anonymous from the nearby guards. The stunt assassination is basically the same concept, except you manually jump in his direction and assassinate him off his horse right when you hit the ground.

Colonial Reinforcements
Colonial soldiers (blue coats) and British soldiers (red coats) attack each other on sight. I use that to my advantage by having the captain killed by the opposing side. For that you need to be detected by the captain first. Being detected will alert the nearby enemies, so it would be wise to take some of them out first. Then lead the captain to the cannon previously shown in the video. If you can get him on it he may jump to the lower beam, then to the ground. Since the captain can easily defeat soldiers without dying, disarm him to change the odds. The soldiers in question are at the fast travel entrance. Now here's the thing: When you reach them while in open conflict, they start to attack you. So I had to kill them afterward.