Ruined House

Sink a hellevator anywhere near the middle of the map and you will come down beside one: a squat tower of obsidian brick standing in the ash, several floors tall, usually with lava sitting in at least one room. A minority are built from hellstone brick instead. They only generate across the middle half of the Underworld, so a shaft dug near the far left or right edge of a world can miss every single one of them. If you have never seen one, that is normally why.

Two things bring people inside. The first is the Hellforge, the only station that will smelt Hellstone into bars, and there are usually a few loose in the rooms. Take one back up rather than carrying ore down and back on every run. The second is the Shadow Chest. Almost every one in the world is inside a tower or sitting on its roof, and their loot pool is the strongest gear available before Hardmode that no boss drops.

Hellstone is also thick in the ash right around the buildings, which is convenient, because you have to be there anyway.

You cannot break a Shadow Chest and you cannot force it. It takes a Shadow Key, and that comes out of the locked golden chests in the Dungeon, which puts Skeletron in front of it. The key is never consumed, so one opens every chest in the world.

That ordering catches a lot of runs out. People reach the Underworld the moment they have a Molten Pickaxe, find six locked chests, and have to walk the whole layer again later. Mark the towers on the map on your first pass and do the circuit properly once the key is in hand. The pool is Sunfury, Flower of Fire, Hellwing Bow, Dark Lance, Flamelash, Magma Stone and Treasure Magnet. Every one is a real upgrade at that point, and the Magma Stone keeps mattering well into Hardmode because it feeds the Fire Gauntlet.

Hellstone brick burns on contact, so in those towers every doorway costs health until you have an Obsidian Skull or one of the accessories built from it. Apart from that, the two building materials behave identically.

Lava is the larger problem. Rooms are flooded at generation and the pools are deep enough to end a fresh character in a couple of seconds. An Obsidian Skin Potion buys you time, but the quicker fix is to mine one block out of the wall at the low side of the room and let the pool drain into the ash. Bailing lava with an Empty Bucket is slow and you will always miss the last tile.

Furniture in these towers ignores lava entirely. Everything except the Demon Torches stays put where normal furniture would be knocked loose, so you can strip a tower down without losing anything to the pools. That matters more for the walls than the contents: a hell bridge for the Wall of Flesh fight wants a few thousand tiles of something, and hauling that down from the surface is a chore. Two or three towers cover it, and obsidian brick crafts into Obsidian Platforms that stay put once placed.

Nothing about them looks ruined. Older versions of the game punched random holes through the walls and the background behind them, which is where the name came from, and the name simply stayed. The current generator still leaves gaps in the horizontal walls, but they are placed so that each room has a way in, and the background lines up behind them so you barely register the hole. The odd half block that turns a doorway into a lavafall is about all that is left of the damage.