Ancient Blue Brick Wall

The wall is a quick crafting step, but the material behind it only exists once you have found Shimmer. Ordinary Blue Brick dropped into a shimmer pool comes back out as Ancient Blue Brick, and that transmutation is the only source. Shimmer sits in the Aether, a single small cave the world generates once, so this is a building material you unlock by going looking for that cave rather than something you carry out of the dungeon itself.

Looks are the entire point. These walls use the dungeon art from before the modern redraw, flatter and simpler than the current set, so the obvious build is one that wants the old game's dungeon instead of today's. The green and pink walls come out of their own shimmered bricks, which gives you all three dungeon colors in the older style if you feed the pool enough of each.

Craftable dungeon walls are the safe kind. Enemies will not spawn behind them and the area does not register as dungeon for biome checks, unlike the natural walls down in the real thing. You can line a whole house or a boss arena with them and still live there.