Placement

Everything you put down lands in one of a handful of layers stacked over each other, and knowing which layer an object belongs to explains most of the placement rules you will run into.

• Solid. Anything that stops you moving. Nearly every block is here, plus a few oddities like Teleporters and Boulders.
• Background. The furniture layer. Plants, chairs, crafting stations, storage. No tile holds one of these and a solid block at once, but you walk straight through them.
• Walls. They sit behind everything else and block nothing, but they support the placement of other objects and they decide whether a space counts as indoors.
• Wiring. Wire and Actuators, invisible unless you are holding a mechanism or carrying a Mechanical Lens. Nothing in this layer collides with anything.
• Liquids. Not placed in the usual sense, but they occupy space and interact with the other layers on their own terms.

A handful of things refuse to pick a side, and those are the ones worth remembering. A closed door is solid and an open one is background, which is the entire reason a door can seal a house and still let you through. Platforms are background objects you can stand on top of, and holding down drops you through them.

Actuators are the general purpose version. Wire one to a solid block and a signal turns that block into something you walk through, then back again. They only work in one direction: an object that is natively background can never be forced solid. The Active Stone Block is the exception that carries the ability by itself with no wiring.

Two more break liquid logic on purpose and are worth filing away. A Bubble holds water back like a solid block while letting you pass through it, and a Grate does the reverse, stopping you while letting the water pour past.

Holding a placeable item and clicking tries to put it where the cursor is. Two conditions have to hold: the item must be flagged placeable, which its tooltip will tell you, and the target has to satisfy that specific object's footprint.

Blocks are the easy case, since they go anywhere adjacent to another block. Furniture is fussier. Most pieces want a flat solid surface of a particular width, some want a wall behind them, a few want both, and a Work Bench needs two flat tiles under it. Torches attach to almost anything. Doors are among the strictest. If you build at all, turn on Placement Preview. It draws the exact footprint under your cursor before you commit, which removes nearly all of the guesswork with multi tile pieces.

Direction matters for anything asymmetric. Chairs, beds and the like face whichever way your character was facing at the moment you placed them, so turn around before you click rather than breaking and replacing afterward.

A pickaxe frees most placed things. Walls need a hammer instead, and anything treelike, meaning regular trees, cacti, giant glowing mushrooms and gem trees, needs an axe. Anything needing an axe is also something you cannot place by hand, so reaching for one means you are dealing with something that grew rather than something built.

Crafting stations extend your recipe list while you stand near them, which is why a good crafting room is a dense pile of stations in one small space. You can also activate a station directly to see only its own recipes, worth doing once the combined list gets unwieldy.

Walls do more work than their appearance suggests. They are what turns an enclosure into a house that an NPC will accept, they suppress most enemy spawns inside a structure, and they give torches, switches and levers something to hang on. A room without them is a room that spawns monsters in it.

Beds set your spawn point. Storage gives you somewhere to dump ore, and a Dresser doubles as the menu for changing your character's appearance.

Two separate stats govern how far you can work. Placement range is how far away you can put a tile down, tool range is how far away you can break one, and several accessories raise one or the other. A few raise both.

Placement range quietly does something else: it extends how far away you can interact with things. Chests, levers and NPCs all answer from further out, and during a build that is often worth more than the extra tile of reach.

Placement speed is the third stat and only matters when you are laying long runs of block or wall. Some items speed up blocks, some walls, some both. Under a certain amount of building it is invisible. Past that it is the difference between a sky bridge taking two minutes and five.

With autopause switched off, you can place an item straight out of an open inventory without moving it to the hotbar. For building that means the inventory stays open the whole session and the hotbar stays reserved for tools.