Wall Mode is a game mode in which walls spawn randomly on the board, blocking off some tiles. A wall spawns every other apple eaten, starting on the first apple. Hitting a wall kills the snake and ends the game. Here are the spawning limitations:
1. Directly in front of the snake - the entire straight line is "safe" - This rule has been removed in a later update and no longer applies.
2. Really close to the snake head in a small radius - this is 3 blocks in texicab distance, the closest it will spawn is considered "avoidable", although it could catch you off guard. Known as the Spawn Radius.
3. Right next to another wall that isn't the board borders - you can't have 2 walls right next to each other.
4. 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.
5. On top of the snake/wall/apple (aka "an occupied" tile).
It was discovered by TheEpicCarrot7 that there are exactly 235,355,155 different unique patterns of walls in Small Board.
The Google Snake Discord Community is working on calculating how many of these patterns have a solution, as of 1st of January 2023, out of half a million random pattern calculated, it seems that 0.11% of pattern have a Hamilton cycle. Hamilton paths haven't been calculated yet, as these take much longer processing time. The current estimate of how long it would take to calculate every single pattern for a cycle or path is over 7 years, barring no major breakthroughs are found in processing time for Hamilton paths.
Yarmiplay made a guide on SRC available here with his general thoughts on this mode.