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

Well, here's a guide to tell you all the known rules of Portal mode. Unlike in every other snake mode, in order to complete the board and fill every space with a snake, you DON'T have to complete what's known as a Hamiltonian circuit/path. https://en.wikipedia.org/wiki/Hamiltonian_path
But you do need to get really close to completing one.

Perhaps you should just brainlessly coil and fill the gaps while playing safe? Actually - yes, with an extra rule - eating apples is dangerous. In case you haven't even played yet, in Portal mode there are 2 apples, eating one will make the snake come out of the other. This will only count as eating 1 apple. In portal mode the snake is also slower, I don't know the exact number of how much slower it is, but I do believe it's faster than slow mode, so between 66%-100% of classic speed.

The speed should make it easier to react in time to close apples that will portal you all the way across the board. Update: This has been changed, now the snake speed is your choice like in all other mode combinations.

Next thing you should know is apple spawning logic - although 100% random, isn't the same as you're used to. Also, end game logic is a little different too.

1. An apple will never spawn in a radius around the snake - 2 block from the head. Apples spawn right when you eat one, so after you portal to the other apple, you can know for sure the marked spaces (in the image) are gonna be without apples. This also slows down the speed running aspect of the game for obvious reasons - you'll never have 2 apples one right after another.
2. When you reach the other side of the portal, you may turn 180 degrees. This is very useful, since you don't need to mind the direction of the snake when going into an apple, as you may change it when coming out of one. Use this to your advantage.
3. The game ends when apples can't spawn anymore, like in every "all apples" category. In case you forget, apples can't spawn in a radius around your head - this means the game will finish before the board is completed. Side note: A score of 252 can't be reached, the game will end earlier.
4. If an apple has only 1 possible spawning position, because all the other spaces are in the radius around your head or are the snake itself, the game will finish. Both apples must be able to spawn in order for the game to keep going.

The main problem with Portal All Apples is pockets and apples that spawn in and out of them as they like, so the strategy reaching late game is to avoid eating the apples (that's right), and clear out the pockets as best as you can. Of course, this becomes much harder later on, where you barely have spaces to move.

It's also a good strategy to leave "escape routes" whenever you can, 2 spaces wide if possible. When reaching extra late game, 200+, you probably won't have that luxury, and an apple might spawn in front of you in an unavoidable position, this is when not having pockets will save your life. Either way, so far no one has reached that point, so I can't really tell you what to expect. As of writing this, my top score is 171. Here's an image from another run though, to give you a look at how late game looks like: As you may be able to tell, I've been taking my time eating the apples, due to the risk involved. Most of the space is open and my tail is reachable, which is how you reliably survive.

The end game (225+) depends heavily on RNG, it's easy to imagine end game RNG that will end the run without giving you a possibility for victory. So good luck out there :)