Actuator

Wire one onto a block and that block gains an off switch. Signal it and the block turns intangible without being mined; signal it again and it comes back exactly as it was. Nothing is destroyed and nothing has to be replaced, which makes this the tool for anything you want to open and close repeatedly. Like wiring, actuators stay invisible until you hold a mechanism item.

The Mechanic sells them once she is rescued, and for most builds that is where the supply comes from. The free ones are already in the ground. Sand traps, lava traps, boulder traps and Dead Man's Chest traps are all built with them, and a Wire Cutter pulls them out the same way you harvest wire. They turn up far less often than wire does, so it is worth stripping every trap you disarm instead of leaving them behind.

You place them by hand or through The Grand Design, which also removes them. The Multicolor Wrench cannot, oddly, even though the wrench is part of the Grand Design's own recipe. The Presserator drops one under every object you place while you are carrying any, and the Actuation Rod flips actuators within a short range with no wire involved at all, which is what you want while terraforming rather than while building something permanent.

An actuated wall is the best entrance in the game. There is no door for a Blood Moon zombie to smash and no opening for anything to path through, and you get in and out with one key. It is the standard way to seal a base for an event or an invasion. Traps work well with it too: deactivating the block a trap sits behind still lets the trap fire, so you can line several up without the front one swallowing the shot.

Deactivating a block is not mining it, and the game checks for mining. That lets you tunnel past Hellstone without the lava behind it pouring out, cross a Hive without releasing honey or waking anything inside, and walk through Chlorophyte long before you own a pickaxe that can break it. The Actuation Rod is what makes this practical, since running wire for every step of an underground trip is miserable. For plain terraforming, clearing a plot by toggling rather than digging also leaves you the option of putting it all back.

Any block directly supporting storage or a heavy fixture stays solid. Chests, barrels, trash cans, altars, hellforges, dressers, mannequins, hat racks, pylons, trees and cactus all lock the tile beneath them, and an empty chest locks it as firmly as a full one. Golf cups are exempt as well, and blocks that already live in the background, like living fire and smoke, have nothing to switch off.

Progression gates apply too. Lihzahrd Bricks below the surface will not deactivate until Plantera is dead, and in front of the temple's unsafe walls you cannot even see or place an actuator until Golem is down. Bubbles refuse to deactivate when the signal comes from a pressure plate, a switch or a lever, but a sensor or a timer works on them fine.

• Anything resting on top of the block, herbs and glowing mushrooms included, is knocked loose or destroyed when the block goes away. Moss fuzz survives.
• Inactive blocks do not count as house walls, so actuating part of a frame quietly evicts whoever lived there. They do still count toward biome requirements, evil biomes included, which is useful for artificial biome builds.
• Liquids and falling blocks like sand and slush pass straight through an inactive block, so it is never a dam.
• Enemies and NPCs spawn on top of inactive blocks and immediately drop through them.
• Portals from the Portal Gun vanish if any block they are attached to is switched off.
• Take the actuator off an already inactive block and the block stays inactive forever. The only ways back are to place another one and toggle it, or to mine the block and set it down again.
• If you are standing inside a block when it reactivates you are not crushed. You can move around the small pocket you occupy and are stopped by everything past it, which is an odd but usable panic room.