Wall Creeper

Wall Creepers cluster around spider nests in the cavern layer. You will recognize a nest before you walk into it: the pocket is packed with Cobweb and lined with a dark, distinctive wall. They wander a short way outside as well, and although they belong to the early game a few still turn up after the world goes Hardmode.

Background walls are what make them a problem. With wall behind them they can hold any surface, including the ceiling directly over your head, and they pass through platforms without slowing down. With no wall behind it, all of that goes away and it just walks at you on the ground. Clearing wall out of a stretch of tunnel is the cheapest way to make a nest manageable.

Their climbing has a limit worth abusing. A narrow two tile shaft is too tight for them to pass, so a thin vertical corridor cut into the nest lets you pick them off from a spot they cannot follow you into. A single missing tile does nothing, since they crawl over gaps that small. Bring something that covers above and below you, because attacks arrive from the ceiling as often as the floor.