During chapter 6 you will run into a town called Bolmus Populi. Head all the way to the right of the town and enter the golden gate. You will talk to a rather rude rat, and then be asked to leave. When you leave you will meet Robert and he will tell you he can get you to the towers in the sky. The achievement will not unlock as soon as you meet him.
After this you will teleport to the Mountain Giant (I forgot his name) right outside of town. He will open up his mouth for you to go inside. You will then have to make your way down to the bottom. YOU DO NOT NEED TO FIGHT ANY BATTLES. You will need to fight the spiders to continue the story, but not for the achievement.
Head as far right as you can in the tunnel and then drop. You will have to avoid one spider. There will be a bridge in the background and a very short cut scene. After this, drop straight down into the bottom. At the lava pool go left and you will be at the door.
The lever combination is Right, Left, Right, Right. Then with Igniculus, hold
while following the spinning lights to light them up. The door will open followed by a short cut scene. As soon as it finishes the achievement should pop.
Good luck, hope this helps!