At one point I modded humans as a playable civ, and had quite a bit of fun with above ground, human forts. The rules are pretty self explanatory, use wood mainly for construction, and all permanent structures must be above ground.
Later, humans became boring, and I reverted back to playing dwarves. However, I just can't seem to get rid of my above ground habits.
Thus was born the fortress of the Great Dwarven Plain.
What it is is basically an above ground styled fort built in a cavern. All the rules for above ground construction apply, except that stone blocks are obviously the main material for building. All farms must still be "outdoors", and all bedrooms, dining halls, workshops, etc must be "indoors" (surrounded in all directions by walls and ceiling). There must also be at least one ramp into the layer of cavern you inhabit to allow caravans.
Also, after the first year, ALL above ground buildings must be deconstructed/abandoned.
The main difficulty would be invading and holding the cavern in the beginning, and customarily all my starting 7 has some degree of military skills.
To help this play style, the world gen is generally set to only have one layer of caverns, and I use reveal in the beginning of the embark to find a suitable location.
It is hard sometimes, especially when you run into blind cave ogres and crocodiles in the first 2 months or so.
It is also fun as hell.