Trap Door

Which way it flips depends on where your character's midpoint sits relative to the door, not where your feet are. Standing under it sends it upward, standing over it sends it downward. That sounds obvious until you are on a staircase half a tile below it and it still opens the wrong way, which is the center rule catching you out.

Furniture on either side blocks the swing and it will use whichever side is clear. Block both and it simply refuses to open, so leave room when you plan a shaft. An open trap door is also too narrow to stroll through off a ramp, you have to jump, and the tiles around it need to be solid blocks rather than platforms. Like a normal door it takes wire, which is the cleanest way to make a floor hatch that opens for you and stays shut for everything else.

Zombies force ordinary doors during a Blood Moon. They cannot do a thing to one of these. Putting the entrance to an early base in the floor rather than the wall is a genuinely effective way to sleep through your first few Blood Moons, and it costs almost nothing to build.