Background walls

Walls are the backgrounds of buildings. All Placed walls prevent enemies from spawning, and a room filled with "safe walls" is necessary in order to attract NPCs or to use a bed to set the spawn point. Note that naturally occurring walls are considered unsafe until the player tears them down and re-places them.

Walls may be removed with a Hammer, but walls that weren't placed by a player have to be torn down from the edge (i.e., there must be at least one adjacent tile with no wall).

At positive elevations, Dirt Walls are the mainstay of the map. Dirt walls must therefore be removed from either the top or bottom edges of the zone of positive elevation, making the construction of safe houses in the center of the dirt zone a fairly lengthy task.

At negative elevations, the backdrop of the map is no longer dirt and walls of the player's choice may be placed on the backdrop freely.

Walls don't have any monetary value, not even those made out of higher tier ores, like Gold.

Anywhere with a wall can be used to place blocks, wood platforms, torches and paintings.

as on version 1.2 (verify?), walls can be "uncrafted" back into respective brick forms in a 4 to 1 ratio (the opposite of the "to walls" ratio)

* - Also craftable.

History
Mur