My current fortress (in year 3). The embark is fairly flat and right next to a small river.
Surface - Walled off connected areas - pastures and butcher/tanner along with refuse pile. These are roofed.
Surface +1 - Barracks for guard, statue garden, fortifications
Soil (-2) - Initial area dug into. Underground farm plot(s), Animal handling, cage storage, Trade Depot, Goblin disposal. The first soil layer is all sand and not built in much due to interference with ditches dug on the surface.
[The Trade Depot connects to east and west tunnels running fairly far out and then hitting the surface- these are still under construction]. These will be the main surface connections besides a narrow postern gate in the main keep.
Stone Layers from here on down. And the basic fortress design is 11x11 square rooms with two doors in each wall. Surrounded by a two-wide corridor and stairs (2 up, 2 down) in the center and each corner. Roughly a 50x50 footprint.
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Level 1 - Storage (trade goods, raw materials from the surface like wood)
Level 2 - Workshops (each 11x11 can hold 9 3x3 workshops with some extra space left over)
Level 3 - Kitchens and Food Storage (kitchen, still, fishery)
Level 4 - Meeting Hall (dining), Hospital,
(turns out this was a marble layer)
Level 5 - Residence Blocks (I use "windmill" design cell blocks - four on the level each centered on a corner stair)
Space remaining off the central stair is used for housing nobles and officials (mayor's office for instance)
Level 6 - Residence Blocks (see Level 5) - not currently dug out beyond the stairs
Level 7 - Facilities - Connection to mining galleries, catacombs, water cistern, also a DWR installation to power a few things.
Below this is shafts downwards to the Magma Sea, a Cotton Candy spire, and weaving amidst the caverns. This fortress hit magma at Level 103, so the actual fortress depth is fairly shallow compared to my last one. Two large rooms at the bottom handle smelting and forging as well as ore and bar stockpiles. Plus pottery and glass since the fortress has yellow sand and kaolinite in abundance.