The Lost City - Starting the Burn Baby Burn Challenge and
Breaking Down the Gate with the Trebuchet

Now it's your turn to rain down a little destruction. The goal is to destroy the gate, but there's also a Challenge called Burn Baby Burn to be undertaken here. Climb up next to the trebuchet and use your axe as a handle to crank the wheel, rotating the platform to the left...

...until the beam jutting out ahead lines up with the wooden signal fire built on top of the glacier.

Hold RT/Left Mouse to charge the shot. A white circle will creep up the cliff, indicating where the trebuchet is aimed. When the circle overlaps the signal fire...

...release RT/Left Mouse to launch a fireball.

If your aim is true, the tower will catch fire, initiating the challenge. Otherwise, try again until you get it.

The goal is to light five of these signals in all. All but one of the signals can be targeted from here. The next signal fire (2/5) is just to the right of the first. So rotate the trebuchet until the beam lines up with the signal. Again, hold RT/Left Mouse to charge the shot. When the white targeting circle overlaps the tower, release the button to fling the fireball.

NOTE: There's another signal in the distance behind the second, but the fireballs from this trebuchet can't reach it. You'll be able to light it later in the level.

The next signal fire (3/5) is a bit farther to the right, behind the gold-domed tower. Aim and fire the same way to light it.

Then crank the trebuchet all the way around to the right to spot the next signal fire (4/5) close to the glacier. Aim and fire as before. You should now have all but one of the signals lit. The last one will have to wait.

Finally, crank the trebuchet back around to the left so the protruding beam points straight at the large gate.

Hold RT/Left Mouse until the white circle is directly over the gate.

Then release the button to hurl the fireball and destroy the gate.