Demiplane Pit Trap?

[Zhorn]

So Demiplane specifies that you cast it "on a solid flat surface"...

Just a hypothetical, but if you were to cast Demiplane on the floor, would the entrance inside the Demiplane then be on the ceiling?

Realistically there's no logical requirement for the Demiplane to have to share the same orientation and gravity direction of the location you are casting from, and it could be a whole "thinking with portals" situation.

I was just looking at this spell and think of ways to use it as a prison. The spell is not concentration, and makes no reference to being able to close the door at a time of your choosing, leading me to assume it'll remain open for the full 1 hour duration. With that assumption I'm just thinking of how to get a creature into the Demiplane and not them simply just walking out before the door vanishes.
But if the door was on the ceiling of a 30x30x30 room, then it's more reasonable to assume they are stuck in there (not accounting for magic, flying, or other superhuman abilities that would allow them to get to a door 30 feet above ground).

[quindraco]

You can definitely put the door in the ground - in fact, you could put it on a sufficiently large shield, and carry it around. It's 100% thinking with portals.

Since the spell doesn't specify, I imagine you could have the door open into the demiplane in any orientation you choose (I've always assumed demiplanes have standard gravity, because too much about the spell breaks down into absurdity if it hasn't got any gravity). Just don't forget the door is summoned in a closed state, and has to be opened. Since it's a door, you should be able to close it. Slap Arcane Lock on that baby, and hey presto.

[Wizard_Lizard]

You could also dispel magic the door away?

[Millstone85]

Just don't forget the door is summoned in a closed state, and has to be opened.

Could you decide that the door opens with a push, then summon it under the enemy's feet?