Portal

The one real skill test in the game. Gold on all 18 Challenges - the tight thresholds on Least Portals, Least Steps and Least Time across chambers 13-18. Bronze and silver (Basic Science, Rocket Science) fall out of tidy play, but gold needs genuinely optimised routes, especially Least Steps and Least Time on the later maps. Don't brute-force the nasty ones - look up a per-challenge gold route and copy it. This is comfortably the hardest achievement here; everything else is a formality.

Earn a bronze medal on all 18 Portal Challenges. After you beat the game, the Bonus Maps menu unlocks Challenge Maps: the six chambers 13-18, each with three targets - Least Portals, Least Steps and Least Time. Bronze thresholds are generous, so simply completing each challenge in a sensible way clears it. Knock out all 18 and you're well on the way to Rocket Science and Aperture Science too. Optional, post-game.

Detach all 33 wall-mounted security cameras. To knock one off, place a portal on the wall directly behind it and the camera pops straight off its mount. They start appearing in the early test chambers, so get into the habit of looking up in each room - a few are tucked above doorways and in corners.

Do them all in a single playthrough. The count is unreliable across separate sessions, so if you reach the end short its cleaner to run the story again from the start than to try mopping up odd ones via chapter select - hence soft-missable rather than a free go-back-anytime collectible.

Beat any two Advanced Maps. Once the story's done, Bonus Maps unlocks the Advanced Chambers - harder reworks of chambers 13-18. Only completion counts (no time or portal limit), so just clear two of them. You'll pick this up on the way to Fruitcake and Vanilla Crazy Cake. Optional.

Euthanise the Weighted Companion Cube. In Test Chamber 17 the game hands you the Companion Cube to weigh down a button, then - to open the exit - insists you drop it into the Emergency Intelligence Incinerator. You have to do it to finish the chamber, so despite the grim name (and GLaDOS's guilt-tripping) its completely unmissable. Pour one out for the cube. :(

Pick up one turret and use it to bowl over another. Easiest in a turret-heavy room like Test Chamber 16 - grab a turret, carry it, and swing or walk it into one of its friends so the turret you're holding knocks the other flat. It has to be the held turret doing the knocking. Quick and optional.

Beat four of the six Advanced Maps. Same as Cupcake - completion only, no medals needed - just two more on top. Optional.

Beat the game - defeat GLaDOS at the very end of the escape. She detaches into personality cores (morality, curiosity, anger, the lot); grab each one with your portal gun and drop it into the incinerator on the floor while side-stepping her rockets. Burn the last core and the ending plays. Unmissable, and the natural end of the run.

Story-forced, so you can't miss it. Up to Test Chamber 11 you've only had a single portal colour to work with; at the start of Chamber 11 you pick up the full dual-portal device (both the blue and orange portals) and it pops the moment you equip it. Carry on as normal.

A single big fling covers it. In any chamber where you can place portals on both the floor and a facing wall, drop a portal on the floor and another high on a distant wall, then fall into the floor portal - you'll come flying out of the wall portal and sail across the room. 300 feet is roughly one good fling in a tall room; the wide chambers later in the game (or the fling rooms in the escape) work nicely. Optional stunt.

The game's dry name for surviving GLaDOS's fire trap. Right after The Final Test (Chamber 19) she starts lowering you into the flames on a moving platform - portal off the platform onto a side ledge before it tips you in. Escaping is how you reach the maintenance areas and carry on, so its unmissable. Welcome to the backstage of Aperture.

Silver on all 18 Challenges (the six chambers 13-18, each scored on Least Portals, Least Steps and Least Time). A clear step up from Basic Science but still fair - clean, thought-out solutions get silver without needing frame-perfect execution. For the tighter maps, glance at a route rather than fighting it. Optional.

Set up an infinite fall and go put the kettle on. In any chamber where you can place portals on both the floor and the ceiling, put one directly above the other, then step into the floor portal - you'll drop out of the ceiling portal and fall endlessly, racking up distance. 30,000 feet is about five minutes of falling, so start it off and wait it out. Optional.

The Portal 2 ARG collectible. Scattered through the test chambers are 26 hidden radios; carry each one to a specific spot in its room and the static resolves into a signal (its light turns green) - the transmissions are Morse code and images teasing Portal 2. You need all 26 lit. Finding the exact sweet-spot in each room is fiddly, so follow a radio-location walkthrough and work through the chambers via chapter select after you've finished the story. Nothing here is time-limited, so its always doable - optional.

Beat all six Advanced Maps (the harder versions of chambers 13-18). Completion only - no medals required, unlike the challenge achievements - but the advanced layouts are properly tricky, so take your time. This finishes off Cupcake and Fruitcake. Optional, and a satisfying last tick.