Does mining into a mountain to make a large area and slapping a door on front count as shelter?
Don’t see why it doesn’t ?
- Moving your flag inside
- Make a tunnel where you can place 3 or more tunneldoors behind eachother.
- Cheap fortress accomplished
Should but … replace the cold stone floor with a nice stone, wood or clay foundation.
This should work … probably
Windows are the only problem you can’t rlly fix down there 
With the new feature of “personal space” you may run into hearthlings getting sad and cramped. ( Currently there isn’t any easy way to make tunnels taller)
Also another thing is that you can’t grow crops underground anymore as far as I know? But there’s nothing wrong with a temporary underground space right now! I use mine mostly for storage piles.