While the in-game editor is a great tool for realising your puzzle idea, it doesn't have the flexibility create anything outside of the pristine walls of an Aperture testing chamber. For that you need Hammer, Valve's Source map editor.
It's a powerful tool, but also complicated and at times downright awkward. That hasn't stopped people from using it to create more in depth campaigns - collections of levels that contain custom visuals, stories, scripting and even special physics rules. Here's five that show off what a dedicated mapmaker can achieve.
1. 12 Angry Tests
12 Angry Tests is an exceptional seven part campaign that serves as a standalone GLaDOS-free story that mirrors the structure of Portal 2. you start out in an abandoned and worn-down Aperture, solving fiendish laser and refraction cube puzzles. From there you wind up in the depths of old Aperture, playing with gels and momentum, before the hard light/excursion funnel heavy final act, among rows of long abandoned test chambers.
It's full of clever flourishes, like one room late in the game that completely reconfigures itself just as you're about to solve it. There are plenty of twists from the plot, too. Like any good Portal 2 campaign, there's a surprising antagonist hindering your progress. In all, it's an hour or so of new content with pitch-perfect difficulty and a keen eye for what makes a great portal-based puzzle.
Download 12 Angry Tests here
Set after the events of Portal 2, Decay shows an Aperture that has been left to... well, you know. Run down test chambers aren't exactly a new idea for Hammer made maps - even Portal 2 itself is filled with them - but Decay really takes the dilapidation to extremes. Corridors are wonky, puzzles no longer align properly and absolutely everything is falling apart.
The puzzles included are some of the hardest on this list. The second of the three parts is particularly tricky, pulling every design trick to introduce new obstacles that disrupt previously sound plans. In the third part, elements of different test chambers overlap, causing plenty of misdirection. But the visual spectacle of the climactic climb through the rubble is a compelling reason to endure its red herrings.
Download Decay here
3. Designed for Danger
One of the early plans for Portal 2 saw Chell at the whims of a selection of personality cores, each with their array of test chambers. Designed for Danger gives you an idea of how this might have played out. You start in one of the main game's early test levels, but as you move to solve it, you're broken out by Rick, the adventure sphere. Apparently Nolan North doesn't make enough appearances in games, so we have to put him in mods now.
Rick has designed a bunch of deadly adventure chambers for you to solve. Well, sort of. Designed for Danger's theme is mostly sold through visual touches and between chamber sections. For the well designed puzzles, danger merely means "contains a lot of lasers." That is, until the final act, which sends you through the bowels of Aperture.
Download Designed for Danger here
4. Curious Chamber
Curious Chamber is a three part campaign that subverts gravitational direction to create a series of puzzles that are constantly surprising. To give the least spoilery example, the first chamber has an uncrossable gap, a white wall to fly out of, but nowhere to build up enough momentum to do so. Except, that when you investigate a small corridor with a target painted at the end wall, gravity flips 90 degrees and you're suddenly falling into it.
Throughout it shows off just what tricks are possible with Hammer. That reaches a whole new level in the last part, a series of chambers that are inventively presented to create a memorable, and funny, experience that becomes increasingly difficult over time.
Download Curious Chamber here
5. Moonbase Luna-C
A series of test chambers set on the moon! That means more than just seeing acres of greyish-brown rock outside of the windows, because these maps also make use of reduced gravity. It seems like such a small change - you can jump higher and for longer, but so what? Except the effect the change has on your ability to navigate levels is massive.
It's a case of relearning your limits and intuitively realising which leaps you can make. But the levels also feature liberal use of laser grids, both below and above you, to create challenges that would be impossible with our stupid, restrictive Earth gravity. Still, no matter where you are, the turrets remain happy to politely kill you.
Download Moonbase Luna-C here