So, you noticed that the "All Apples" category in Wall mode is empty, and now you started to think "maybe I'll be the first to take that spot, free mainboard WR."

Well, here's a guide to tell you all the known rules of Wall mode.
First things first, in order to complete the board and fill every space with a snake, you must complete what's known as a Hamiltonian circuit/path. https://en.wikipedia.org/wiki/Hamiltonian_path

Unlike every most other modes, where you can just brainlessly coil and fill the gaps while playing safe, wall mode puts obstacles in your way.

Every odd number of apples, a wall will spawn - if the spawn logic allows it. What's the spawn logic? Well, a wall can't spawn in the following situations:

1. Really close to the snake head in a small radius - here's an illustration: https://media.discordapp.net/attachments/732814593037959269/876584896515170364/canvas_3.png?width=1023&height=904 The closest it will spawn is considered "avoidable", although it could catch you off guard.
2. Right next to another wall that isn't the board borders - you can't have 2 walls right next to each other
3. In a way that makes the borders into a death trap - generally, if an apple spawns in a corner or next to the border, the closest walls to it will always allow the snake to get in and out safely as long as you play right.
4. On top of the snake/wall/apple (aka "an occupied" tile)

These are all the rules for wall mode. Strategies? Assuming the goal is All Apples (lower categories are just "book it", maybe expect for wall 100) You should play as safely as possible and attempt to fill up the board in a way that leaves 0 pockets and a path to the tail for yourself. This is pretty difficult as it is.

But even if you manage to do that perfectly, you could still run into a pattern of walls that make the ham-path impossible. In fact - most of your attempts will die to this. There's nothing that can be done about that as of now, besides resetting and hoping for a pattern that allows the ham-path - those patterns are known to exists. Another strategy is to place the snake body while you eat the apples in such a way that will not allow wall spawn to destroy you ham-path. Pulling this is seriously difficult, since you have to focus on surviving and the snake never stops moving, but it's still possible, I'd even suggest this is probably mandatory, because if you don't do this, walls are likely to spawn in bad positions killing the pattern and your run. And that being said, it only takes a few really bad walls to kill the run.

If you ask me, as long as RNG is this likely to just kill your run without even giving you as much as a chance, going for wall mode all apples normal size is a fool's errand.

Wall all small board is possible and has been achieved on 1 apple and on 5 apples, but it does take a lot of grinding and luck.

Now that you that know the rules and strategies, Good Luck!