Walls have a 1.5 block tall hitbox, meaning players and mobs cannot jump over them, unless the mob is a horse, has Jump Boost, or is standing on a slab. A wall block becomes a post when there is no block directly next to it. If a block is placed next to it, they will connect to each other, extending their hitbox horizontally. Like fences, spiders can still climb over a wall block.
Only cobblestone and mossy cobblestone walls are currently available. However, after the release of update 1.14 in early 2019, walls will be available in each of the following materials:
Walls are made with a 1:1 ratio with their corresponding material; this is different from fences, where the ratio is 1:1.67.
|Wall Block x6|
A wall's primary use is enclosing an area. It functionally serves to keep mobs and players either inside or outside. This is because walls are 1.5 blocks tall (although they appear to be only 1 block tall). A large area can be made resistant to hostile mobs, by enclosing it with an unbroken wall, and lighting the inside portion heavily. This keeps any monsters from spawning within the walled area, and keeps most hostile mobs that spawn outside, from coming in. A notable exception are spiders, which can climb over a wall. However, wall can be "spider-proofed" by placing a normal block on top of a two-block-tall wall. The spider will hit the bottom of the block when climbing the wall, and fall back down. Walls cannot always protect players from skeletons either; While a skeleton is unable to cross a wall, it can still shoot arrows over it. This risk can be lessened, by making a wall two or more blocks tall. torches can be placed on top of walls, creating a well-lit, safer perimeter.
Walls can also be used as railings, especially on the edges of balconies, or floating platforms. Since walls cannot be jumped over accidentally, this keeps players and mobs from falling off, while still allowing them to easily see past the wall.