Is there a guideline for the number of individuals who can inhabit a structure? For instance, if I build a Keep that costs 100k (assuming it is if basic construction and average size for the price) how many dudes can live there?
Domains At War states that unit capacity for garrisoning equates to 1 unit (120 people) per 1,000 SHP for a stone stronghold. One can estimate a stronghold's SHP by dividing the value by 8 - so this 100k structure has about 12,500 SHP, and should be able to garrison 12.5 (13 rounded up) units.
I think, and hopefully Alex chimes in, that 'garrison' in that context means either a rotating bunk system or everybody's huddled up during siege, and therefore represents a very uncomfortable (to modern eyes) maximum.
My house, which more or less equates to a "Cottage, Wood", has a listed unit capacity of "1", or 120 people. If we're including the basement, I could probably garrison 120 people in the house if they're not all sleeping at the same time - I wager I could get ~30 twin bunkbeds in here to sleep 60 without giving up the master bedroom or my office, though I don't expect the kitchen or the bathrooms to keep up with demand.
City code, however, says I can only have 12 people living here, which sounds outrageous to me, (though that's more the question of affording 10 children...) so I'd go as far as to say "modern comfortable living" tells us that you can only garrison 10% of the regular unit capacity if you're living like many modern Americans do - having multiple rooms and such available to hang out in and leave empty, which 'back in the day' was kingly.
So, a standard 75,000 gp keep could garrison roughly 9 units (1080 men). Assuming a tenth of that is normally stationed here it would have room for 108 men. Is there a standard proportion of soldiers to support staff?
Indeed there is!
Each 120-man unit requires 2 armorers, for equipment maintenance. Alternatively, an armorer with 2 journeymen and four apprentices can support 240 men.
Each unit also requires a quartermaster.
Optionally, each unit can have a lieutenant, and given average leadership ability, there's probably at least two division commanders and an overall general for those 9 units.
This is all from Domains At War: Campaigns; Military Specialists, pg 23.
I'm relatively confident the "unit" count is an abstraction containing the proper support staff for each unit, and you don't have to reduce the capacity of the stronghold by the number of the support staff implied - i.e. those ~4 people per unit and the extraneous commanders are abstracted away.
To clarify my earlier post, I can't "garrison" with 10% - my house would be woefully underdefended and would last maybe 1 round in a siege - I'm making a guess as to the value of the space that is utilized in day-to-day activities, rather than during a defensive action.
The rules are kind of bent towards the utility of a structure during conflict rather than the day-to-day existence, so, one has to make guesses. There's a bit of abstraction going on that doesn't necessarily take into account the purpose of any part of the structure versus it's value in unit-count.
If you have Sinister Stone of Sakkara, that's a really great example of a working stronghold and the living space available within for different classes of individuals. The legate's Villa is a structure that can garrison 1 unit, and there's a HQ that can garrison 2 units, but in day-to-day use they maybe have 10-20 people coming through, and less staying there. It's a ~179K fortress, usually garrisonned by 600, with only 90 troops at the time of the adventure. Since it's so undermanned, there's a lot of thoughts given on what that means, and what it would entail to fully stock the fort.
Even if the adventure is never used as written, the section on the fort is worth the purchase as an instructional guide on applying ACKS' stronghold rules out from a abstract concept into a working habitation.
How many stories does the standard 80’ tall Keep have (the one that is 75k and measures 60 x 60 x80 tall)?
I think the presumption is a story every 10 feet, that's how walls work, at any rate - though higher ceilings are certainly an option.
So, a keep that costs 75,000 gp would have approximately 20,000 sq. ft of interior space.(assuming the interior dimensions are 50 x 50 and not 60 x 60). If we extrapolate that it can hold one person per 100 sq. ft. under day to day living conditions that means it would be fully staffed with 200 men: soldiers and their support staff (armorers, cooks, grooms, etc.), plus officers, healers, administrators, some carpenters and masons to take care of repairs, and so forth.
In times of war the Keep could hold a full 9 units, although that would be pushing it to capacity and would be unsustainable over a long period of time.
Does the above sound reasonable?
As comparison, the "infantry barracks" from Sakkara is 140x30, one story. It's made up of apartments, or "blocks" of living space. 12 blocks of 300 sq ft per 10 regular soldiers (30 sqft/man), 1 block of 300 sq ft for four "sergeants" (75 sqft/man), and then then 300 sq ft for the captain of the company at the end.
That's one unit in 4,200 sq ft - dividing that out, that's ~4.7 units that can fit in the 75K keep in that style.
If they're hot-bunking or whatever, then I expect 9 units could very uncomfortably exist together for the duration of the siege - though I couldn't make a guess as to where they'd keep their supplies.
That being said, the 75K keep is marked as holding 12 units - that's 14 sq ft per man, or 28 sq ft per man if they're hotbunking. That follows the sq ft Sakkara's fort gives it's regular soldiers, but it's beyond me how the ones who are awake don't end up standing on the ones that are asleep.
Submarines are the most cramped thing I can imagine in modern-day war machinery, and I've been inside the U-505 boat in Chicago. Those beds probably weren't any more than 30x60, if that - that's about 13 sq ft, stacked two high. Stacked 3 high in 10 ft ceilings, I bet you could get about 150% more troops in the Sakkara fort's barracks at least, though it'd suck. Bunk the sergeants and commander together in a block, and that's 9 blocks, or 2,700 sq ft per company - that gets you 7.4 units in that keep, which doubles to 15 - so 12 units would at least leave some room to walk around.
Sakkara's fort has 6 units worth of "barracks" - then 38 units worth of capacity from other structures and the walls around those structures - presumably the other 38 units would be living in the other buildings and tents within the walls in a siege-y sort of situation. A keep by itself with no other support structures attached, or a wall around it to make a yard, would probably only see 9-12 units in a do-or-die sort of situation.
The Castillo de San Marcos in San Agustin was besieged in 1702 by English ships sailing from Charles Towne. The fortress is 50,000 square feet. In its current state, about 10,000 square feet of that is courtyard, but at the time, the chambers were smaller and the courtyard was larger - after the siege, the courtyard shrank so that the tops of the walls could be 40 feet wide in order to host larger guns (the walls themselves vary from 12 feet thick on the landward side to 19 feet on the waterfront). It housed the entire population of the city plus garrison (approximately 1,500 people total) for two months (roughly October 27th to December 29th). Even if they camped in the courtyard, it's only 33 square feet per person, and they had cannon, gunpowder stores, and enough supplies that the siege wasn't succeeding after two months.
That's interesting to know. I've decided to use 100 sq. ft. per person during normal conditions.
As you've already surmised, unit capacity is not a good guideline for continuous comfortable habitation. Unit capacity is primarily intended to tell you how many soldiers can man the walls, and how many can assault the walls at once.
The Romans had highly-efficient barracks and I think 30 square feet per soldier would represent a good basis for long-term habitation in an ancient or medieval world.
The Romans were also careful to make sure that officers and nobles had much better accomodations. A rule of thumb might be that living space is 30 square feet + (Level-1)^2 x 15 square feet. So a 3rd level character would get (30+ 4x15) = 90 square feet, a 5th level character would get (30 + 16 x 15) = 270 square feet, and a 9th level noble would get (30 + 64x16) 1,054 square feet. This is roughly similar to what the Romans/Aurans offer.
For a more "knightly" feel, where cavalry are given much more luxurious accomodations than the Romans offered, one might say that soldiers get their wages x 2.5 in square feet. So light infantry get 15 square feet, heavy infantry get 30 square feet, and heavy cavalry get 150 square feet.
Servants lived under similar, if not worse, conditions. Many servants in a castle or fortress didn't even have beds and simply slept on floors or benches in halls, kitchens, and other large rooms.