Zip down the line and you will walk into the ruins. Try to remain on rooftops only, because the streets are being patrolled by the Deathless.
Over the rooftops, head towards the gate that you can see in the distance (the screenshot).
You can also try to eliminate several lone guards. However, you need to stay vigil here and act stealthily, best by attacking them from above.
After you reach the gate, it will be shut close and you will get under fire from the trebuchet at the other side of the location.
It is very difficult to reach the tower with the trebuchet, because you will have to face the Deathless on your way there. After you kill them all out, use explosive arrows to eliminate the archers near the trebuchet.
To get near the trebuchet, you need to climb along the right side of the tower's base. Prepare your path using two arrows and drive them into the wooden surface.
Jump up to reach the other arrow and from there, jump towards the ice wall. Remember to jab the hatchet into the wall.
Using the hatchet, turn the trebuchet to make the long projecting beam point towards the gate at the other side of the location.
Press RT button to adjust the force of the shot. Hold it down up until the white crosshair aligns with the gate, and release it. The trebuchet fires a fire ball that destroys the gate. Return to the gate and walk through.