I tend to build forts in fits and spurts of mining that cause it to graduate over major size categories
Young Fort: Exists on 7Z, give or take
Z0: The Surface. A 3x3, 5x5 or up to 11x11 if I'm feeling generous perimeter of constructed walls. One side features a 3-wide raising bridge
Z-1: Entrance. The topmost soil layer consists of a 3-wide passage to a trade depot, the stairs down, and a grid of 9 3x3 farm plots
Z-2: The Hoard. a single 11x11 room on the first stone layer
Z-3: Industry. 5-7 3x3 rooms off a single passage and a quantum stockpile for stone
Z-4: Housing. 8-12 "Private" 1x3+door rooms
Z-5: Food. 11x11 generic Food stockpile, still, 11x11 room with tables and chairs to be a dining room, butcher's shop, tannery, and refuse stockpile
Z-6: Administration. 4 3x3 rooms. This houses offices and quarters for nobles that need such, and levers: at this point, just the front gate lever.
Usual CoD for Young Forts: Goblin Ambush before progressing to Middle Fort
Middle Fort:
A starting Fort that has a 1x1 stairway down to Magma, possibly with twisting tunnels where it hit the caverns, and those cavern ends sealed off with hatch covers. A mass of smelters, forges, kilns, and glass furnaces are constructed over the Great Magma Sea. Housing is expanded significantly and food minorly, mostly in its food and refuse stockpiles. Industry gets 1-2 extra workshops and a wood stockpile. Crypts consisting of 1x1+door graves are constructed on Z-7. Military is constructed on Z-8 consisting of a 11x11 Barracks and a 4x3 Danger Room if desired.
Usual CoD: Ambush-induced tantrum spiral after military is drafted but before it is properly equipped.
High Fort:
A middle fort with either most of Z-1 or an extra soil layer (between Z-1 and the Hoard on the first stone layer) hollowed out into a tree farm. massive expansion of stockpiles, housing, food, and administration. Industry doubles in size of workshops and gains specialist stockplies. Military expanded with an archery range and hospital, while Z-9 gets an ammo stockpile for bolt return at the very least.
Z-9 or Z-10 have prisons built. Massive walls may be erected to "Claim" more surface area, especially if above-ground farming is desired or tree farm is insufficent. Population becomes too high to bother micromanaging most of them, and FPS begins to drop.
Usual CoD: None. High Forts are marked by stability: the Fortress progresses to its High phase once it's safe from goblins and (most) megabeasts. I've yet to have a High fort visited by a Bronze Colossus, though.
Late/Degenerate Fort
A High Fort that begins to construct pointless megaprojects and semi-megaprojects. Usually above-ground constructions or the like pyramids and towers, but sometimes underground multi-Z halls or mixed-level engineering projects like pointless pump stacks. The Late period is usually ushered in by claiming the Monarch and ensuring his/her happiness. Degenerate forts get boring because the challenge of grand projects is drawn out by the slowness of 300 dwarves on the case, where more dwarves has actually become less dwarfpower-per-second.
Usual CoD: FPS-Death, followed by Magma-Related ‼fun‼, flooding, and trying to mess with the caverns where nearly a dozen syndome-spewing FBs have been allowed to build up.
Middle Forts are probably my favorite stage, with 60-100 dwarves: enough to have surplus dwarfpower without starting to sap my FPS or will to care about a one of them, though the stability of High Forts can be fun while they're still running quickly and growing towards the monarch.