Background walls

Walls are tiles that create backgrounds, most being usable as items. Natural walls make up the background of most biomes, and placed walls are an essential part of a house. 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. Naturally occurring walls are considered unsafe until the player tears them down and re-places them (if they can be harvested).

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 of version 1.2, walls can be reverse-engineered back into respective brick forms in a 4 to 1 ratio (the opposite of the "to walls" ratio).

Naturally Occurring Walls
These walls are spawned when the new world is created, and can all be obtained by some method.

Unobtainable Walls
These walls are spawned when the world is created but are unable to be created, obtained, looted, purchased, or used by the player in any way. They can be destroyed, but drop nothing.

Purchased Walls
These walls are purchased from various NPCs.

Converted Walls
These walls are created naturally when Corruption, Crimson, or Hallow spreads to them, or using the respective Solution with the Clentaminator. When destroyed, these walls do not drop anything.

Crafted Walls
These walls are made by the player using various crafting materials.