A triangle foundation attached to your entrance wall, two door frames on adjacent faces, a solid wall on the right. The doors swing toward each other. You cannot have both open at the same time.

What It Does

The outer door has its hinges on the left and swings right into the triangle. The inner door has its hinges on the right and swings left into the same space. Both open toward each other, so if one is open it physically blocks the other. You close one before the other will move.

Without an airlock a door camper waits outside and walks straight in the moment your door opens. With one, the worst they can do is follow you into the triangle. They are stuck there with you, not past you into the base. It costs almost nothing and goes up right after your TC.

Wood doors are fine early on A wood door holds up at the start of a wipe. Satchels, beancans, shotgun shells, and fire will all get through one, but that takes resources most people are not spending on a fresh base. Upgrade to sheet metal when you have the frags, not out of panic.

Why a Triangle Foundation

A triangle is exactly half a square. The two door frames sit on adjacent faces: one on the shared wall with your base, one on the left face when you are standing inside looking out. The right face gets a solid wall. That puts the two doors at roughly a right angle, which is what lets them physically collide when both are open.

On a square foundation the outer door swings inside the base and the inner swings outside. They never meet. Use a triangle.

How to Build It

1
Lay your base foundations first
Build your square foundations as normal. Pick your entrance wall before touching the airlock. Face it toward open ground, not a cliff or rock you can get cornered against on the way in.
2
Snap a triangle foundation to the entrance wall
Select the triangle from your building plan and attach it flush to the outside edge of your chosen wall. It should sit neatly and point outward.
3
Replace the shared wall with a door frame (inner door)
Remove the twig wall between the square and the triangle, then place a door frame. Hinges go on the right.
4
Place a door frame on the left face of the triangle (outer door)
Standing inside the triangle facing outward, the left face gets the outer door frame with hinges on the left. The right face gets a solid wall.
5
Hang both doors with hinges facing each other
Outer door: hinges left, swings right into the airlock. Inner door: hinges right, swings left into the airlock. Open both at once to confirm they collide in the middle before upgrading anything.
6
Add a ceiling
Without a roof someone drops in from above and both doors are bypassed entirely. Triangle ceiling, upgraded to stone with the rest.
Diagram: outer door hinges left, inner door hinges right, solid wall on right face (add after in-game screenshot)

Common Mistakes

Square foundation instead of triangle. The outer door swings inside the base and the inner swings outside. They never collide and the airlock does nothing.

Door frame on the wrong face. The outer door goes on the left face of the triangle when standing inside looking out. Get them switched and the hinge directions cannot be set correctly.

Wrong hinge direction. You have five minutes after placing a door to pick it back up and re-hang it, and you can rotate door frames with a hammer regardless of building grade. There is no reason to leave it wrong.

Test before upgrading Open both doors at once. They should jam against each other in the middle. If they both swing freely the hinge direction is wrong somewhere. Fix it while everything is still twig.

No ceiling. Someone on high ground or a second floor drops straight into the triangle and skips both doors. Roof it.

Upgrading

Upgrade the airlock to stone with the rest of your base. The triangle foundation is 150 stone, the ceiling another 150, two door frames at 300 each, one solid wall at 300. Around 1,100 stone for the whole airlock.

Sheet metal doors are worth it once you have the metal. A wood door will hold at the start of a wipe but satchels, beancans, shotgun shells, and fire will deal with it when someone puts in the effort. Stone walls with a wood door is still a real barrier, just not a permanent one.