Reasons to encourage and codify "base-building in civilization" in the mid-levels:
- My players generally buy a townhouse anyway and then complain about lack of base-building, so I think if there were reasons to build more stuff they would happily do so.
- Generalize to cover wilderness basebuilding / refuge-upgrading with the same rules, at a distance-and-danger cost multiplier.
- Base facilities could help ease the pain of small markets.
- Maintenance cost for your base would be a nice way to streamline / centralize cost of living expenses.
- A collective party possession encourages unity and the party as the unit of continuity.
- Provide absolute clarity on what it means to get the treasure "home" for XP purposes, especially in points-of-light settings where there isn't a big "civilization" zone just off the hex-map.
So here's how I see this going down in practice.
Find a town you like. Go meet with the local lord and petition for the right of hearth and charter. He's going to ask you to do him a favor. It's going to be a messy, adventurous favor. If you have powerful enemies in town (from, say, a night of drunken mayhem fighting temple eunuchs - purely hypothetically, of course), he's going to ask you for an additional favor per powerful enemy. If you have a powerful ally in town, maybe he asks you for one less favor.
So you deal with his ankheg problem and come back, and swear a little oath to help defend the town from threats monstrous and domestic, and he authorizes your charter as a Legitimate Fraternal Order (like the Elks or the Masons - make sure to choose a name), granting you the right to build or purchase a hall within his domain. You go find some crappy land on the outskirts of town and hire some laborers to clear it and build a longhouse. Congratulations, you are no longer murderhobos (just murder-...citizens). Everyone now knows where to find you - traveling merchants, barbarians for hire, messengers bearing quests, peasants looking for help with their wyvern problems, your wizard's roommate from college, your wizard's ex from college, knights errant looking for someone to fight, beggars, preachers, used magic item salesmen, performing acrobats seeking a venue, the thieves' guild seeking to acquire something while disguised as acrobats, the thieves' guild looking to move some hot goods, the watch looking for some hot goods, the tax collector, the assassin's guild, meddling archmages, elder dragons... I think I feel a random table coming on.
It comes with enough space, kitchen, and larder enough to feed and sleep n people. This is the maximum number of PCs, henchmen, hirelings, and retainers you can support in "clean, sanitary conditions" suitable for healing and the avoidance of loyalty penalties. Upgrade for more capacity. If you have excess capacity, townsfolk come drink and gamble with you and some of them might be recruitable as henchmen. Some of them might also be spies for the other Rival Adventurous Orders with halls in town.
Also comes with some capacity for horses and dogs; again, if you want more (or cavalry retinue), kennel and stable upgrades.
Other stuff to put in or around your hall (eventually compound):
- Library and laboratory for magic research. Capture a spellbook from an NPC with new spells in it? Goes in the party library, becomes guild secrets.
- Chapel, sanctum, meditorium, summoning chamber, etc for spell point recovery.
- Treasury / vault / reliquary. Stop paying those bankers negative interest to store your gold, and store it yourself.
- Monster heads on the walls / trophy room. Bonus to reaction rolls on your own turf.
- Maybe a more general "prestige / grandeur" mechanic? Helps negate the hiring penalty for slander?
- Armory. Keep your magic swords organized, and stockpile plate mail when it's available in your lousy market.
- Cemetery / catacombs. Inter your dead henchmen properly to prevent them from rising as the undead for vengeance, and to keep high-level wizards from using their skulls as crafting components (keep the skulls for yourself). Spend money on elaborate funerals for your PCs for more efficient reserve XP generation (per Heroic Fantasy Handbook). Ghosts give you quests.
- Shrine to your dead fighter, Bob. Subsequently play a cleric of Bob. Bob becomes the Party Deity. Praise be unto Bob.
- Smoke-filled back rooms in which to plan hijinks and dungeon-crawls, away from the prying ears of Rival Adventurous Orders and The Law.
- Pit where potential henchmen can fight each other so you can learn their stats. Or for gambling. Or both.
- Warehouse space to store your trade goods, both stolen and legitimate.
- Wagon yard for assembling your caravan(s), might be visited by traveling merchants or halflings if space available
- Psychoactive - regain spell points in the field, but save vs datura
- Ornamental, +prestige
- Go on, plant that magic seed you found
- Infirmary with physician hirelings to cure your lycanthropy and succubus-herpes without it becoming public knowledge. And more natural healing, I guess.
- Forge with smith hirelings to make more plate for your henchmans. Or masterwork weapons, if those rules are in use.
- Still for Dwarven Brewing.
- Gem-cuttery for lapidary hirelings to improve found uncut gems (hat tip to Courtney's Downtime and Demesnes, the draft of which I should probably finish reading before attempting to roll my own thing here)
- Parade / training yard to upgrade your mercs.
- Siege workshop / testing range... I feel like this might be pushing the edge of the Charter, though, and start generating Concern from your local lord.
- Dock / shipyard, if coastal or riverine.
- Menagerie of exotic beasts
- (Prestige again, but a small chance of escape...)
- Barn and pasture of not-so-exotic beasts for use in trapfinding, hecatombs, and rations-on-the-hoof
- Dungeon / prison for beastman prisoners and hostages
- Walls, towers, etc... but again, local lord will push back on too much of this.
All of this has been done before, in for example the 3.x Stronghold Builder's Guide. All this is aso easy enough to work out under ACKS' rules as they exist. It's just in a very low-abstraction state at the moment, much like Domains at War. You're hiring individual mercenaries and buying individual windows for your hall. This is an easy enough problem to solve.
Bonus: use the same facilities rules for NPC organizations like wizards' cabals, temples, and thieves' guilds. Membership in a guild at low levels provides access to libraries, laboratories, and restoration facilities in exchange for dues and labor during downtimes. These organizations are chartered differently and don't compete with adventuring organizations, but have monopoly on sale of eg magic in town (nudging players towards adventuring rather than sitting in town selling spells).