After talking to your men at the inn, you'll start the mission on a rooftop with a musket. Next to you will be Charles and the others will be on the ground by a wagon. They are going to overturn the wagon to stop the military caravan coming through and when they stop, you are going to open fire. Start firing at them and take out as many as you can to help your men on the ground. Soon, they will all be dead and you'll rejoin you friends.
Slipping into something a little more uniform, you'll take up position driving the carriage next to a Native American woman. While riding, you'll watch a couple of your guys off to the left take out a couple of guards on the road. Up ahead, you'll have to instruct them to do so by hitting the "assist" button (L1/LB). Whenever guards may take notice of you, use this button to take them out before they become fully suspicious. Even in instances where it seems like your allies would be noticed, they won't be so feel free to use them as often as is necessary.
The wagon will leave you at the front gate of the fort with your party. Walk along with them but don't go too far ahead because an officer will approach you from the distance and you have to wait for one of your friends to nab him from the nearby haystack. With him out of the way, you need to free the prisoners nearby and perform three corner assassinations. While you may be tempted to start with the nearest groups of prisoners, they are easier so it is best to start with those on the ship.
Prisoners
Head directly left and go into the water. Swim around to the far side of the ship and start climbing up. Just before climbing in, watch for the guard that's pacing around the middle of the ship. Wait for him to go up by the prisoners. When he does, take cover behind the crates in the middle. Stay here and approach the right corner on this side of the crates. When you do, you'll lean into to take cover. When the guard walks by so you can see him, press the B/O button to whistle him over. When he investigates, kill him with a single attack. Now go to the front of the ship and quickly kill the two guards there with either one or two separate assassinations. Finally, go to the back of the ship and take another corner position. Whistle him over and assassinate him. You now have two corner kills and you just need to save these prisoners here.
Head for the prisoners by the water now. You can jump into the water nearby to get there or you can make your way over land, which ever seems easiest to you. Come out of the water near the prisoners but be careful of the officer that is patrolling nearby. You may not see him immediately but he is definitely there. When you've found him, wait for him to pass by to one side and walk into the bushes nearby. You'll crouch in the bushes and eventually, the officer will come by. Kill him when he does and then free the prisoners here.
The last set of prisoners is going to be across the road. Cross over and go past the prisoners. There will be a couple of tents around here. Stand behind the tent nearest the wall and take up a position at the rear corner closest to the prisoners. When you see the officer patrol walking by, whistle to him to bring him over. When he comes to investigate, assassinate him to get your third corner assassination. Lastly, before freeing these prisoners, take a look around the camp. Switch on Eagle vision to find the general as a golden outline. With him out of the way, free the last prisoners.
Silas
With all of the prisoners free, you'll finish killing a nearby enemy and a firing line will form in front of you. At this point, you'll be prompted to defend yourself from it. Press the A/X button to use the nearby enemy as a human shield. When you've done so, close in on the firing line to take them out.
The fighting will continue indefinitely so you need to start making your way towards Silas. You could make a break for him and just start running but it's very likely that you'd get hit by a firing line and lose your constraint. For this reason, it's best if you try to slowly make your way there in combat so you can avoid firing lines as they form
You'll have to make your way to the wall in the distance and use the boxes nearby to climb up. At the top, Silas will be defended by a group of red coats. Fighting them should be no different from any other fight. When you are done, track done Silas to end this mission.
