Portal Stories: Mel, Anna Mieke, also playtested it before its final iteration. She made me a very helpful video playthrough (fast forward to 14:20) with live comments and incredibly valuable criticism that greatly improved the quality of my final design.
The critical path does not contain any shortcuts, but it does contain small variations in solving certain puzzles. Playtesting proved to be immensely useful in this regard since testers would find their own ways of playing the level, so I could see which paths and solutions that I wanted to keep. The difficulty level is right where I wanted it, because players who had little experience with Portal 2 needed only some help while players who knew the game a little better did not need any at all.
Early feedback by Anna Mieke
Key areas and puzzle highlights
The first room of the level contains three glass cubes, a laser beam, and a turret robot that the player has to take out to advance. This puzzle can be solved in two ways:
The first solution involves using all three glass cubes to redirect the laser to a white surface on which a portal can be placed, then using the portals in combination with the laser to burn the robot.
The second solution is to only use two of the cubes to make the laser hit the white surface, followed by, once again using portals and the third cube to correct the skewed angle of the laser.
Retrieving a companion cube
In the second room, the player is in need of a so called "companion cube" to activate an orange gel dispenser. It can be obtained in a third room right next to it. An unlimited supply of cubes can be dropped from a dispenser, but they drop into a tractor beam that pushes them into disintegrating fields.
The solution is to step on floor buttons to temporarily turn off the fields in timing with the cube's flight through the air. Once the cube hits the white wall, the player can use portals to make the tractor beam push it back to the other side and onto a ledge.
To finally get hold of the cube, the player must first temporarily disable another disintegration field by pressing a button, then use a portal to move the tractor beam, which releases the cube and drops it onto a trampoline.
Early playtesters found ways of cheating by keeping one portal where the tractor beam hits the white surface and leaving the room. This short puzzle below prevents that by only making it possible for either a cube or the player to leave the room without passing through a disintegration field.
As shown earlier, cubes that hit these fields are destroyed. But when a player passes through it all portals they placed are destroyed instead. The player has to stand on a button that disables the field and drop the cube onto a trampoline that bounces it to the other side of the field.
Flying with orange gel
Back in the second room, the player activates the orange gel dispenser. There are a few different surfaces to experiment with portals in the area, but only one of them takes the player closer to the exit - an area high up in the ceiling.
This area is just one long ledge. The player can use the white areas around it to create a long line of orange gel. The orange gel can be used to run very fast, creating momentum for the player to "fling" themselves into new areas using portals. This mechanic is thoroughly explored in the game's main campaign, and the area is designed to remind players of how they have used it before.
The fling mechanic can be used to fly through the air in another section of the room. Using the angled panel that can be seen from the ledge now covered with orange gel, the player can reach a platform they could not get to before.
Bouncing with blue gel
The platform leads the player to a floor button that dispenses huge drops of blue gel while the player remains standing on it. The player can use portals to make the blue gel fly in the same way they just did using the angled panel. I set the blue gel dispenser to use these huge drops in order to symbolize what the player must do next, use the blue gel's bouncy properties to reach another new area.
The big leap
Using the ledge covered in orange gel from before, the player can fling themselves once more. This time, the blue gel makes the player bounce further than the platform they reached the first time, sending them flying towards a trampoline on the wall.
With the height gained from the trampoline, the player must turn around and place a portal on the other side of a wall they could not see past until now. The trampoline not only bounces the player high enough to pull this off, but also throws them straight back at the angled panel which can be used to end up where they just placed their other portal.
If they could not pull this off, they can use any other white surface to transport themselves over the wall.
The blue/orange finale
The final puzzle involves using the companion cube to open up a white wall panel near the ceiling. At this point, the cube is powering the orange gel dispenser, so the player has to retreive it while keeping one portal in the final area to get back over the big wall they just leapt over.
Some playtesters fetched the cube, went back to the blue gel and splashed it on the floor in the final area, then stepped through a portal on the white wall panel to bounce up to the exit.
Other playtesters did not think of using the blue gel and instead placed the cube back in the orange gel dispenser after discovering the white wall panel. They then painted the floor in the final area with it, brought the cube back to open the white wall panel, and ran to the exit. I decided to keep both solutions so that players can clear the level using their favorite color of the two.