Core Keeper - NPC Housing Requirements & Tips (Comprehensive List)

Debunking the ubiquitously incorrect information on this topic, to help those struggling, by using results of careful in-game testing in Core Keeper Version 0.3.5-05e7.

Requirements for NPC to Teleport into a Room

(1) Fully enclosed by walls of any type.
(2) Summoning item – “Slime Oil” or “Mysterious Idol” (from defeating bosses).
(3) Bed – crafted (not “Caveling Bed”).
(4) 1 empty tile of space – that includes no torches.
(5) Minimum floor space of 4 tiles (e.g. 2×2), as dictated by the above requirements.
(6) Max size 63 tiles (i.e. 8×8 – 1).
(7) Their existing bed is picked up (if there is one). Picking up only the summoning item or breaking a wall/door is not sufficient.
(8) Player moves away so that new room and NPC themself are entirely off the mini-map for some amount of time. 10 seconds seemed to do it for me, but it may be inconsistent, so try going further for longer if stuck.

Not Required

(a) Door.
(b) Light source (NPCs have their own).
(c) Specific floor covering or wall type (natural dirt will suffice).
(d) Separate room – NPC will share, provided there’s enough beds.
(e) Clean bed – it *can* be player’s respawn point.
(f) Entirely solid floor – holes are fine, provided enough good floor space.
(g) Safety – room may contain enemies and their spawn surfaces (e.g. slime).

Advanced Tips and Tricks

(i) Placing the summoning item in the doorway can let you past while blocking them in. They seem to have a bigger collision box. But it must be a side door with a clear tile above the Slime Oil/Mysterius Idol. Other furniture can be used to force the NPC’s position in bigger rooms.

(ii) Spoiler for late game: NPCs can be summoned to the wilderness, then (given their immortality) used as a punching bag to hit a crit and trigger your Soul of Azeos Thunder Beam, which is able to break through Mould Dungeon walls.