Walls.

Heibai really only consists of two platforms. One you can land on... and one's that kill you.  Of course, the eraser-tools in the game which kill you are more-so obstacles than "platforms" but you get the picture anyway. For a while, these were literally the only two pieces of geometry I used to make every level in the game - this was until we decided to add walls to the levels. We did this, initially, because I thought they'd be useful in being able to lock to player into certain areas by physically blocking them from being able to travel in one direction. However, they didn't end up really being used in that way. Sure, there's plenty of examples in the game where a wall stops you from being able to skip sections of levels or forces you to jump in one direction over another (like the beginning of level 9 for a quick example) but in general walls are much more commonly used to make jumps for interesting.

What makes a jump interesting is down to a few things. It relies on how you use the three core pillars I've talked about before, sure, but it also comes down to how much a jump challenges you to think. Some jumps are simple, you just tap space, hold a direction and let go of that direction when you're close so you don't jump too far and miss. But other jumps require a mixture of inputs and thought that you can work out before and during a jump, using the intuition you've gained from your experience with the game so far in order to complete the jump. This is why I used the walls in the way that I did. Jumping between a platform and a platform, a platform and a wall, and a wall and a wall creates variety in the jumps in Heibai - not even mentioning sprinting on top of that - and that variety makes the game's platforming fun the play and keep it engaging. 

Of course, as I've hinted at before, this breaks the core pillars of Heibai: Walls are thinner and harder to land on, they create barriers which the player can crash into and they make the arcs of the jumps messier. With the last one in particular, it really is a lot harder to telegraph a jump with background assets and the placement of the buckets in the scene when the player doesn't know if they're landing on a thin wall or a regular platform, especially when the player is jumping almost straight up to reach the top of a wall or the like. You can see examples of this all over the game, like in level 7 where the player must jump straight up in order to hit a bucket which activates a platform directly under yourself allowing you to reach the wall - but there's two walls! One in white and one in black. One is supposed to let you jump over to the first star and the other is to prevent you from getting into the star path without jumping over to the first star in the first place (or at least until you've hit the bucket in front of the first star). So these sorts of jumps are designed to challenge you to experiment or think beforehand because players aren't expected to work something out like the above example straight away, especially in a casual game aimed at a younger audience like ours. It doesn't matter that they break the pillars, as long as it's not egregious, because it's expected by their design that they behave differently in a level once you learn about them (it's why they're not much of an obstacle in 6/7/8 as you're being eased into the different situations they'll be used for).

In the end, the walls in Heibai are a good example of how something intially designed to do one thing can be used for something unexpected and end up being a core part of the level design and gameplay experience. Enjoy!