Do WALLS matter if I'm building the 6-inch brick defense from Corruption/Crimson for my city?

Question

I'm aware of the Tiles required to be 6 inch thick brick for Corruption/Crimson not to spread. But does the walls (dirt walls, underground walls ...) matter?

Answer 1

As far as I'm aware it doesn't matter. Corruption and Hallow (and the other spreading biomes) only spread through blocks, and compatible blocks (Dirt at Normal (and at contact), Stone, Sand, Ice and Grass at Hardmode) at most.

Think that, if these biomes spread through walls, you would have to demolish them. And in the upper layer of the word that may be unfeasible, as you have to start on top or at the bottom. That and, due to the mechanics inside the game, only blocks are calculated.

IIRC, the limit is 3 blocks^{1 2}, not 6, for Corruption to spread.

Answer 2

The simplest way to block Corruption/Crimson/Hallow spread is to make an opening of 3 blocks, across which they cannot spread. Once you have a fast pickaxe (or drill) and wings, it becomes very easy to dig vertical channels to stop horizontal spread, and horizontal channels to prevent vertical spread.

You can buy Purification Powder from the Dryad to clean up small areas of Corruption/Crimson even before hardmode. You can also use Corrupt Powder to cleanse Hallow.

Once you get a Steampunker NPC, you can buy a Clentaminator + Green Solution to cleanse large areas of all three, which can be used very effectively to completely expunge the Corruption/Crimson/Hallow in a contaminated area. I have cleansed an entire world of them, then re-seeded a few isolated areas so I can still access the monsters for each of those biomes.

Corruption/Crimson/Hallow spreads through plain Stone blocks, grass, sand, and ice. They will also convert jungle grass to regular grass, and mud to dirt in order to spread to the grass.

In hardmode, one affected block can spread to other blocks up to 3 away, so a gap of 3 blocks wide/tall prevents spread across the gap. BUT, if there is Corrupt Grass (only Corruption, not Crimson or Hallow), then thorns can grow and corrupt across a gap. To prevent this, destroy any Corrupt Grass or cover it with something other than Dirt blocks.

Also in hardmode, even after you've seemingly cleansed an area, it can become reinfected over time by a single block. Because breaking altars with the PwnHammer can cause an infected block to be randomly generated in the world, I recommend against breaking any altars after you begin cleansing your world with the Clentaminator.

Answer 3

From the wiki:

[Corrupted/Crimtane] will not spread the [Crimson/Corruption]

So no, they will not, though they still count towards the Dryad's percentage

Wiki: https://terraria.gamepedia.com/Crimson_Wall