The tree idea sounds good. Or if I can find a big rock somewhere. And earth walls too.

Those Gjall raids concern me, but if natural earth walls work against land attack, perhaps "earth ceilings" can work against air attacks...

Considering starting off with a Hobbit hole by building a shelter, and then terraforming land above the roof as natural protection. Then expanding sideway/downwards into a luxury Hobbit hole or Dwarven fortress. Just need to find a way to get sunlight in while keeping that roof... And keeping framerate decent...

No idea if it'll work but let's see