Spike

Spikes are most of why the dungeon punishes sloppy movement harder than anything else before hardmode. They only react to players, so nothing in the room will set them off for you, and the hit is tracked separately enough from normal damage that you cannot ride invincibility frames across a floor of them. Dangersense lights them up through walls, which is worth a potion on a first descent.

Dodge effects will cancel the contact damage, but the bleeding lands anyway. In the dungeon that means clearing the next room with your regeneration switched off, which is far worse than the hit itself. Mine them out rather than trying to thread between them.

They count as walls for housing, so a spike lined room is a legal place to put an NPC if you want the look. Explosives clear a floor of them in one go under normal conditions, though the For the Worthy seed takes that shortcut away and leaves you swinging a pickaxe.