Background walls
Background walls, also just known as walls, are tiles that create backgrounds on top of biome backgrounds. Walls are hidden behind everything else, such as blocks. Some types of walls are naturally generated at world creation, being abundant in certain biomes like the Underground Jungle. They are one of the main components a Terraria world consists of alongside blocks and liquids. The player can also place walls manually on any tile which does not already contain a wall, given that it is adjacent to another wall or block. Walls can be removed with a hammer or explosives.
Safe and unsafe walls
Placed walls are always considered either "safe" or "unsafe":
- Safe walls prevent enemies from spawning in front of them (but do not do anything to prevent their passage), and a mostly unbroken background of safe walls is necessary for a structure to qualify as an NPC house, or to use a Bed to change a player's spawn point.
- The vast majority of player-placed walls is considered safe.
- Some naturally generated walls are also considered safe, e.g. in structures like the Pyramid.
- Unsafe walls will not prevent enemies from spawning and cannot be utilized for housing.
- They have a quirk in that they must be removed by starting from their edge (i.e., adjacent space with no wall), which for explosives means that an edge must exist somewhere within the radius of the explosion.
- Most naturally generated walls are considered unsafe.
- Walls that are converted via biome spread or the Clentaminator/Terraformer are also considered unsafe, even if they were previously safe.
- Very few items allow placing unsafe walls by the player.[1]
There is generally no way to distinguish safe walls from unsafe walls. Many types of background walls can exist in both safe and unsafe versions when placed, and these are visually identical. Destroying a wall and replacing it ensures that it is considered safe (unless the player uses one of the few items that place unsafe walls[1]).
Only placed walls can be considered safe or unsafe. Walls in item form are neither safe nor unsafe.
Types
Crafted walls
These walls are made by the player using various crafting materials.
Purchased walls
These walls are purchased from various NPCs.
Harvested walls
These walls can be harvested in various places when exploring the world. Most of them can also be obtained through crafting.
Converted walls
These walls are normally created by biome spread. They can also be made using solution sprayed with the Clentaminator or Terraformer, and on the 
Desktop version, 
Console version, and 
Mobile version they can also be crafted or purchased (with the exception of the natural Mushroom Wall). On the 
Old-gen console version and Nintendo 
version the Clentaminator is the only way the player can obtain these walls.
Notes
- Dirt Walls generally completely cover the background at Underground depths above zero (see Layer, Depth Meter). Placing new walls in this area would require first tearing down the existing walls starting from the top (the Surface) or the bottom (zero depth), which can be an arduous task. Alternatively, the existing walls can be destroyed using explosives.
- At depths below zero, the backdrop of the map is generally no longer covered with actual wall tiles, though this can be difficult to differentiate visually since the biome background at these depths can have a similar appearance. Walls can be placed freely here by players.
- Torches and several other furniture items can be placed directly on background walls without requiring blocks below them.
- Most walls have no coin value, even if they were crafted from rare materials. Stained glass and wallpaper bought from the Painter are exceptions.
- Walls appear to have their own placement layer.
See also
History
- Desktop 1.4.4: Added Ancient Brick Walls and allows the player to place some walls as Unsafe walls.
- Desktop 1.4.0.1: Made several wall types craftable that were previously unobtainable through the use of Ecto Mist.
- Desktop 1.3.1: Most wall inventory sprites updated. Which ones were not are currently unknown.
- Desktop 1.3.0.5:
- Living leaf walls are valid for housing
- Desktop 1.3.0.4: Fixed walls not framing properly in camera mode snapshots.
- Desktop 1.3.0.1:
- Bug fixed: The Blue Slab Wall is now valid for housing if placed by a player.
- Added Honeyfall, Granite, Smooth Granite, Marble, Smooth Marble, Chlorophyte Brick, Shroomite Plating, Martian Conduit, Luminite Brick, Meteorite Brick, and Crimtane Brick walls.
- Many naturally generated walls have a new appearance.
- Now you can place walls in front of each other.
- Desktop 1.2.4.1:
- The Sail now costs 50 instead of being free.
- A bug affecting the appearance of placed Tin Plating Walls (view) has been fixed.
- Desktop 1.2.4:
- Adding walls made from Gemspark Blocks, Palm Wood, Boreal Wood, 1.2.3-Heavy Work Bench-products, and Glass+Confetti.
- The Pirate now sells the Sail.
- Two natural cave walls (7 and 8) have been added. Gemstone Cave- and Moss Cave-walls have received an overhaul.
- The Metal Fence has been split into Lead and Iron variants.
- The Lavafall Wall and Waterfall Wall are now crafted like most other walls.
- Painted walls show up properly on the map.
- Desktop 1.2.3: Adding Dynasty Walls, Lavafall Wall, Waterfall Wall, even more Wallpaper, and the Portable Cement Mixer for even heftier wall placing.
- Desktop 1.2.2: Ten new holiday-themed wall types called Wallpaper can now be purchased from the Painter.
- Desktop 1.2.1: Fixed an issue causing walls to not have different frames. Pumpkin Wall, Hay Wall, and Spooky Wood Wall added.
- Desktop 1.2.0.3:
- Lihzahrd Wall added.
- You can now craft living wood walls.
- Snow particles will no longer spawn on backwalls.
- Desktop 1.1: Added Glass Wall, Planked Wall, Mythril Brick Wall, Cobalt Brick Wall, Mudstone Brick Wall, Iridescent Brick Wall and Pearlstone Brick Wall.
- Desktop 1.0.6.1: Walls that are automatically placed to fill small gaps are now correctly taken from the player’s Inventory.
- Desktop 1.0.6: When placing background walls, one-block holes are now automatically filled in.
Footnotes
- ↑ 1.0 1.1 The following walls are considered unsafe when placed by the player: Infested Spider Wall, Cursed Blue Brick Wall, Cursed Blue Slab Wall, Cursed Blue Tiled Wall, Cursed Pink Brick Wall, Cursed Pink Slab Wall, Cursed Pink Tiled Wall, Cursed Green Brick Wall, Cursed Green Slab Wall, Cursed Green Tiled Wall, Treacherous Sandstone Wall, Treacherous Hardened Sand Wall, and Forbidden Lihzahrd Brick Wall.
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