Here is the full list of all 15 Portal achievements. It takes around 8-10 hours to unlock all of the achievements on Windows.
Fall 30,000 feet.
Beat two Portal advanced maps.
Acquire the fully powered Aperture Science Handheld Portal Device.
Do whatever it takes to survive.
Make the correct party escort submission position decision.
Complete Portal.
Jump 300 feet.
Beat four Portal advanced maps.
Beat all six Portal advanced maps.
Earn bronze medals on all Portal challenges.
Earn silver medals on all Portal challenges.
Earn gold medals on all Portal challenges.
Detach security cameras from the walls.
Knock down a turret with another turret.
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