Go upstairs. Don't turn right at the first possibility but choose the stairs in front of Garrett. Stop where the flames appear, wait until they disappear and go to the other side. Pick up Ancient Armlet - 32G, wait for the flames to disappear and QUICKLY use the switch what stops the flow of gas. Now you can go downstairs and choose the side path, you've ignored previously.
Climb on the highest shelf and find the Scroll Holder - 12G. Now you can open the door. Approach the ladder and get onto the higher shaft level. Continue climbing helping yourself with a claw and pick up Ancient Ingot - 17G you find on your way. When you get on the highest level, hide in the shadow and wait until guards on the other side of the crate move away. Jump on the highest shelf and jump down to the room with the burning oil lamp. Land in the non-lit place.
From now on, start avoiding enemy with a torch, cause only he patrols this area. You can also rob him what gives you two Coin Purses - 9+3G. You find also a Syringe - 10G in the side rooms. And you don't have to even look into the room with sleeping guard because there is nothing interesting. When you search this area, move left from the start and find the place where Garrett may slip in between the chests. You get to The Hidden City - Storeroom. Take a Choke Arrow from the chest and use the switch to unlock an access to The Hidden City - Graven Lair.