Run up the walkway until you spot the busted door on your left, fire a portal on the wall in here and then another on a nearby wall. Pull the lever and then as you approach the end of the tall Aperture Science entrance aim your portal gun at the tall white wall in the top right-hand corner. Now fire one at the nearest white wall and slowly step through.

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Walk around the metal girder to your left slowly until you're directly over the white floor panels below. Now look to the slanted wall panel to your right and set a portal here, now drop straight down and as you approach the floor, fire off the entrance portal, sending you flying through the exit portal and into the wall leading to the next puzzle.

Drop down the walkway, enter the lift and then hit the button to take you to the very top. When you step off outside, move to your left and look for the lit angled wall in the left-hand corner.

MISSABLE ACHIEVEMENT/TROPHY ALERT! After placing the exit portal on the angled roof, place your entrance portal on the wall at the top of the elevator and slowly step out onto the walkway. Press each of the buttons here for the first half of the 'Door Prize' Achievement/Trophy¶

After placing the exit portal on the angled roof, place your entrance portal on the wall at the top of the elevator and slowly step out onto the walkway. Press each of the buttons here for the first half of the 'Door Prize' Achievement/Trophy.

Pull the lever in the control room and fire a portal at the wall directly opposite the - temporarily - locked gate. Once the gate opens, stand by the portal and look up to the far left corner of the room and place the exit portal up above the walkway. Once outside, hang a right down the blue gel flow tubes and then enter the force field elevator to the start of the chambers 'proper'.