Obsidian Brick Wall

The panels already standing in the Underworld ruins are the unsafe kind. A hut thrown together inside an intact ruined house does not stop anything from appearing next to you, and it will not register as valid housing either. That catches out plenty of people setting up a camp on a hell bridge who assumed four walls and a roof were the whole requirement.

The fix is to break them and put them straight back. They drop as items when knocked down, and a wall you placed yourself counts as safe: no spawns behind it, valid for an NPC room, identical to look at. The alternative is crafting the wall from Obsidian Brick, which is usually faster than dismantling a ruin, given that you are already hauling obsidian and hellstone around down there. Either way the item in your inventory is the same one, so there is no version to hunt for or avoid.