Once you're all the way through, you'll reach the final campsite of the game. Take cover, unlike against the Deathless Soldiers, more often due to the ranged attacks, but move between cover when you can since they throw grenades. Your shotgun and assault rifle are the most effective. Use your explosive and poison arrows as well as any Greek Fire. Once at the top and across, you'll have to fight along the path against Trinity mercenaries. Make your way to the tower that leads up to the highest part of the city, watching out of enemies, falling ice, and debris on the way. Scale the walls using your broadhead arrows and the climbing techniques you've learnt. Run through the gate and take down the enemies beyond. From there, create another zipline directly across from the roped pole. You'll have to take out some enemies in the way until you reach the end of the roofs. Use your rope arrow to make a zipline to the lower area where the archers were, progressing over to the gate. Use the siege weapon to take out the archers before breaking down the gate. Run and jump onto the ornament as fast as you can to swing it into the ice, breaking it apart and freeing the trebuchet (13:00 in the video). Then, make sure the ornament is lined up with the ice and release the water in the jug. Pull it under the waterfall to fill it, raising the end with the metal ornament. Then, use your rope arrows to close the jug on the lever. First get to a good vantage point on the far side of the upper level. Once there, you'll need to do a similar thing as to how you broke the large door down in the mine. Make your way over there, using your rope arrows as needed. You'll have to use the trebuchet on the other side for your second shot.
Then, turn it to the gate and the debris will become dislodged. Then, go to that platform and use your rope arrows to connect the pole to the debris jammed in the trebuchet. Once there, aim it first at a nearby ledge with a roped pole. You'll need to swing and climb the structures to get to it. Once cleared, make your way through the door on the level you fought on and go up to the trebuchet. Otherwise, use your shotguns for the heavily armoured ones or your assault rifle to break their armour and your fire arrows to set them alight.
Use poison arrows and Greek Fire when enemies are in groups. You'll have to fight a few waves of enemies here, so try not wasting ammunition. Head left to the next gate where you'll get armour-piercing arrows. Make your way back and go through the gate. Get up to the trebuchet, kill the guards, and use it to break down the gate. Once in the far right corner, use your broadhead arrows to create a climbable path up the walls. There will be lots of heavily armoured soldiers, but most will only use melee attacks. Once there, though, you'll need to head back and up to the right where the trebuchet is. Use the zip-lines to get all the way across the old city to the gate, making your own if needed.