|(note people at bottom for scale)|
So there are these things called Washingtonia filiera or Washington palms and that is some terrible colonyist name for them but hey.
Anyway they shed their leafs and it all builds up in a big ass skirt thing.
There is (on some? all?) a crawl space between the the trunk and the palm skirt. It's a cool , dark , quiet and dusty and a occasionally throughfare for all kinds of life that lives in the skirt
(tarantulas, rats, wasps, scorpions , some birds I think, my research here was pretty light)
Now the thing about the skirt is that it can slough in big heaps , crushing and suffocating anything caught in its way.
Let's imagine a gianter palm, like a 100 stories , with the barks forming stairs and platforms , and occupationally rooms and niches in the every present hatches. And a variety of vermin and flattened life cable of moving freely through the hatch.
Or just keep it how it is, with something humanoidish making nests in the skirt, accessible by climbing up the trunk, but with the danger of movement or activity bringing a section down with you or ontop you, pressing you against the trunk but still kept aloft. Maybe the nest is firmly attached to the trunk , so sections of the skirt can be defensively sloughed as an ablative fortress.
Climbing on the outside is extremely risky as there is no handout that might not give away with no notice
(Note as is these skirts are highly flammable. Accept, adjust or accommodate this as a g.m)
A dungeon in a house. But in the house, a series of interlocking false ceilings, crawlspaces, wall cavities and slender rooms, the house and the secret house interlaced completely, each traversable without realizing the existence of the other.
An easy said than done approach is to start with a isometric building plan and carefully start moving walls and dropping ceilings and blocking off corridors until you have the second house.
The Dead are still here:
The astral and the ethereal planes are generally described as space limitless.
Let's go the other way. The astral plane is contained in anything solid and rectangular . Its astral dimensions vary. Sometimes the same size sometimes 10 fold.
The dead might stay in the walls. A round room is one the dead cannot watch you from. The astral plane only maps to these spaces. Astral forms squeezing through walls and along floors , underground tracing a box around the unalterable curves of a cave. A series of acute angles suggests a curve and these are astral significant as traps and portals.
Ethereal space is maybe a 2 dimensional shadow world painted on surface of everything. Except inside. That's the astral. It hurts to travel from the astral to the ethereal.
EDIT: I just realized how much this owes to "Foundation" from China Mieville's short story collection "Looking for Jake". So shout outing to that right now.