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A portal is the opening that links two rooms, or a room and the street. The game draws the interior through portals, so every room must be reachable from the street through a chain of them. The walls open a hole where a portal sits, with no extra step.

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Connect two rooms

1

Draw two rooms that touch

Rooms drawn almost touching count as sharing a wall. Give them different names.
2

Click Portal

Click Interior (1), then Portal (2). Click on the wall the two rooms share, then click again further along the same wall. The opening is the stretch between the two clicks.
3

Check the direction

The new portal is selected. Its direction reads … → …, from one room to the other. Press F to flip it, or click Flip direction in the sheet.
The opening is 2.2 m high, or the room height if that is lower, and at least 0.1 m wide. Hold Ctrl to skip snapping. If a click does nothing, it was not on a wall shared by two rooms.
Direction matters. An inverted portal loads in game, and the interior disappears from one side of the door.

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Open the interior to the street

Click Entrance (3) and click twice along an outer wall. The portal goes from the room to the street and is marked Entrance. A structure with rooms needs at least one Entrance. A portal to the street always runs from inside to outside, so Flip direction is disabled for it. A physical door is a prop attached to the portal: see Props in structures.

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The Portal sheet

Click the portal in the tree. The tree lists portals by an internal id, not a name.
Portal sheet with direction Room to street, Entrance on, Width and Height, and closed Appearance and sound and Flags sections

An Entrance portal: Room to street, Entrance on, Width and Height.

Portals also have handles in the view: corners, left and right sides, height, and base.

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Mirrors

A mirror is a portal with the Mirror flag on. Turn it on in Flags, and set mirrorPriority when two mirrors can be in view. The other Mirror … flags are described in Flags. A room also has Mirror Potentially Visible.

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Openings and windows

Click Scissors (4). Choose Opening in the bar and drag a rectangle over a wall, a floor, or a ceiling face. The hole appears as a row under the part. An opening with no portal is only a hole: the game draws the sky and fog through it. To make it a passage, select it and use the Opening sheet. With several openings selected, Set … portal(s) converts those whose rooms could be guessed. Pick the rooms by hand for the rest. For a window that faces the street, use Set portal, then turn Entrance off. Cut editable geometry in the bar also cuts the structure’s own geometry where the hole passes. See Geometry.

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Cut or split a room

Switch the Scissors bar to Plan. Start an outline on a room, click its vertices, and press Enter to close it. The area inside the outline is removed from that room. Use at least three points, without crossing edges. Backspace removes the last point. If what remains falls into several pieces, the largest keeps the room’s name, portals, and props. The others become new rooms named … (2), … (3), with no portals. Connect them with the Portal tool, or the Checker reports them as unreachable.

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Troubleshooting

Use Entrance on an outer wall. The limbo is the street.
The room cannot be reached from outside. Add portals that chain it to a room with an Entrance.
The portal to the street is inverted. Delete it and make it again with Entrance, clicking along the wall from inside.
Turn Entrance off, or point the portal to the street.
Delete the portal and make it again between two different rooms.
The room was deleted. Delete the portal or recreate the room.
A corner was dragged off the plane. Make the portal again.
Resize it with Width (m) and Height (m), or delete it.
A warning: the portal is not on the wall. Move it onto the wall.
The portal crosses a corner or overlaps another opening. Move it inside one wall.
A warning. Select the opening and click Set portal. Turn Entrance off if it is a window.