Start the level by going forward.
Climb up the pole.
Shoot some bats.
Get down to the floor, and climb the scafolding on the right. (left in this image).
Kill some more spiders.
Take the rock and throw it out the door.
Put it on the switch.
Go back to where you came from, climb up again, but when you reach the two doors take the other one, and do the same thing with the other rock.
Now go and use the switches so that the light falls exactly on the angels form. It's easy.
The door will unlock. But wait. There's more. When you turn the opening mechanism, it's on a timer.
Climb up the column with the light source.
Grapple, and go through the door.
Kill some spiders.
After a short cutscene, you'll be attacked by two creatures. Kill them, but so that they don't revive, you'll have to go near them and press E, to stump them.
Push the column in the position shown
You can pick up an artifact if you want.
There's an artifact nearby so pick it up as well.
Jump back and you're out.
Like all good things the gauntlet will start working shortly after you exit.
Look into the dark area as you go through the tunnel, you should find a jar there. You can break it and pick up the artifact.
Use the switch to open the door .. and surprise, you'll get to play the prologue .. again. You can use that area walkthrough if you are in trouble.
After this part a short cinematic will follow to explain the situation .. and you'll need to get into the lab. Use the fireplace margin to climb up.
Jump onto the pole and then jump up again to get on top of the glass. Jump inside the lab and you're done.
Enjoy the awesome cinematic ... and be glad you get to see our good old doppleganger.