When mountain homes were more then just a hole in the ground, they had a very obvious layout pattern that was based on 8x8 groups, which were repeatedly used in 16x16 setups. I believe this included the entry, but it seemed to differ, so presumably there were different entry types or were dynamically generated. Also, the only time an entry was "elevated" was if it was located on an incline or steep hill; the game probably thought "Oh, THIS is ground level" and then raised the "top" of the entry to match, just like with those elevated rivers and such.
From my memory, one corner of the level below the bottom floor of the entry pit looked like this:
####....
####....
####.#..
####....
........
..#..#..
........
........
And when copied and rotated/mirrored to fill a 16x16 square:
####........####
####........####
####.#....#.####
####........####
................
..#..#....#..#..
................
................
................
................
..#..#....#..#..
................
####........####
####.#....#.####
####........####
####........####
And then the game would drop the 'ramp' design onto it:
@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@@
@@@@@^^^^^^@@@@@
@@@@@######@@@@@
@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@@
The "@" does not actually exist as a tile, it's just there to get the ramp to the right position, which would be:
####........####
####........####
####.#....#.####
####........####
.....^^^^^^.....
..#..######..#..
................
................
................
................
..#..#....#..#..
................
####........####
####.#....#.####
####........####
####........####
And then a hallway that came off of that would look like:
################
################
################
################
................
..#..#....#..#..
................
................
................
................
..#..#....#..#..
................
################
################
################
################
Of course there were various alterations to the grouped setup, like how it would react if a set of rooms needed a branching hallway:
################
################
################
################
................
..#..#....#..#..
................
................
................
................
..#..#....#..#..
................
#######..#######
#######..#######
#######..#######
#######..#######
And the rooms that came off of that branch usually looked like:
#######.
#.....#.
#.....#.
#.......
#.....#.
#.....#.
###.###.
........
#######..#######
#.....#..#.....#
#.....#..#.....#
#..............#
#.....#..#.....#
#.....#..#.....#
###.###..###.###
................
................
###.###..###.###
#.....#..#.....#
#.....#..#.....#
#..............#
#.....#..#.....#
#.....#..#.....#
#######..#######
But may have had an alternate that depended on the surrounding terrain the same as the branching hallway:
#######.
#.....#.
#.....#.
#.......
#.....#.
#.....#.
########
########
#######..#######
#.....#..#.....#
#.....#..#.....#
#..............#
#.....#..#.....#
#.....#..#.....#
################
################
################
################
################
################
################
################
################
################
tl;dr
It was a modular system that used 8x8 groups assembled into 16x16 groups, 9 of which would fit into a single local map tile (48x48) per z-level, and was limited to a 3x3 local map tile area (ex: 3x3 embark size) If you recognized the patterns the mountain home used, you could replicate that in your own fortress construction. The style was also usually an inverted pyramid; wide at the top, narrow at the bottom.
One of the funny bugs about this is that the surrounding map features were generated after the fortress, so that you'd sometimes find shallow fortresses had "dug" into a pond, flooding an area with pond water; or outright breached a feature like a chasm, river, or pool.