"The Tooth Factory" is a level I made using the in-game editor tools for Terry Cavanagh's puzzle platformer VVVVVV.  The central mechanic in VVVVVV is that instead of jumping, the player uses the spacebar to flip gravity upside down, like so:

This mechanic is explored through a wide variety of puzzles and obstacles.  However, for the purposes of my level I wanted to focus on using two things: spikes and conveyor belts.  My intention was to create something extremely fast paced, which would feel almost like playing a rhythm game in nature.  I also wanted it to be as difficult as possible while still feeling conquerable.

If you want to play the level yourself, the process is a bit involved.  Go here to download VVVVVV Make and Play Edition, and here to download the actual file for "The Tooth Factory".  Open up VVVVVV, select "Player Levels" from the main menu, and follow the instructions onscreen as to where to put the level file.  Once you've put it where the game tells you to, find "The Tooth Factory" on the list of player levels and you're ready to go!

Room #1

In this first screen, I wanted to ease the player in gently while also giving them a view of the level's endgame.  As such, the top half of the screen contains the final section of the entire level, an optional and very challenging bonus collectible.  I wanted to do this because I felt it made for a nice, intimidating preview of what's to come, and because I personally find it satisfying when levels come full-circle, so to speak.

However, all the player has to worry about at this point is getting across the room's lower section.  This is more or less a tutorial section; I wanted to give players who've never picked up VVVVVV before a relatively straightforward place to get a sense for how conveyor belts and spikes work (the former move you in their direction unless you actively walk against them, the latter kill you on contact).

Room #2

In the second room, the player finds the first of the two friends they'll need to collect to complete the level (the green guy on the far side of the screen); I felt this made for a good point of transition between these first two relatively easy rooms and the more difficult ones that follow.  Additionally, there is a checkpoint immediately upon entering the room; I decided to put checkpoints at the beginning of every screen, as the level felt far too frustrating otherwise.  This room is only a slight ramp-up of difficulty from the first; the margins of error are narrower, but the basic gist is the same.

Room #3

This room is where the level starts to get difficult.  Here, the player enters the screen already standing on a conveyor belt and moving towards a spike; it's likely that this will kill them the first time they play the level, but since they've just received another checkpoint it's more of a lesson in being prepared than a real setback.  As a matter of fact, there's no stationary ground to be had from this point on.  I wanted to give the level a sense of momentum, so I decided that after finding the first friend, every surface would be a conveyor belt; as such, the player is always in motion, whether they're following the belts or fighting against them.

Additionally, the room gets harder as it goes on.  The top layer isn't actually so different from what the player has already seen, other than the surprise at the beginning.  The middle layer requires more precise timing, and ends with a narrow drop; this leads directly into the faster up-and-down section of the bottom layer, which has much smaller margins of error than anything the player has encountered thus far.  It also ends with the first of several spike-lined "chimneys" which the player has to freefall through.

Finally, there's an optional collectible in the loop between the top and middle layers.  I added this in because I felt this room's jump in difficulty might become frustrating to someone playing it for the first time; giving them something relatively easy to go after and accomplish makes that difficulty a bit more palatable.

Room #4

This is the most difficult mandatory room in the level.  The first chunk of it is much like the last room, though the "jumps" the player has to make between floor and ceiling are faster and narrower.  Halfway through, the player comes to this conveyor belt:

Unlike the belts the player has seen up to this point, this one is moving opposite the direction they need to go; if they don't actively move against it, they'll be dumped onto the spikes.  From this point on, the player needs to take into account which way the belts are going as they jump between surfaces. 

The third layer up requires that they change their direction of movement mid-jump, as each belt moves opposite the next one in line.  That said, the third layer also has wider sections of belt; in my first design for the room these were just as narrow as the ones below them (if not narrower), but after trying it out I realized that this crossed the line from "challenging" into "miserable", so I eased up a bit.

Room #5

In a sense, this is the last room in the level; the second and final guy the player needs to find is right there, with no other obstacle in the way.  However, for players who haven't felt sufficiently challenged thus far, there's one last bonus collectible to go after.  I made the entryway to this last optional section another spike chimney, because I wanted it to look as dangerous and foreboding as possible to indicate what's ahead. 

Room #2 (Again)

This section is very short, essentially just a bridge between the first and last screens of the level.  I wanted to loop back through the second room as well as the first because I liked the idea of having as much of the level be visible from the start as possible; it was also just the cleanest way to tie everything together.

Room #1 (Again)

This last room is essentially where I put everything that felt too cruel for the mandatory parts of the level.  The jumps between belts are even narrower than in previous rooms; furthermore, the orientation of the belts themselves doesn't follow any sort of pattern, meaning that the player has to memorize how they'll change their momentum during each jump.  Finally, the drop onto the final belt requires virtually perfect timing, plus a very careful midair momentum change so as not to hit the spikes on either side.  After getting the collectible, the device to its right teleports the player back to the final room so they can complete the level.

I felt it was important to have a section of the game that was as difficult as I could possibly make it, because there are always people who want that level of challenge; I also felt it was important that this section be entirely optional, because that sort of experience isn't for everyone.  Regardless of whether the player chooses to go after the last collectible or not, my hope is that they find the level as a whole a challenging but ultimately a rewarding experience.