Base building mini-game

I've been playing a lot of Subnautica lately.  It's an open-world exploration game, set 90% underwater.  It has beautifully rendered environments and creatures that really sell it as a completely alien planet.

A big part of the fun is the base building mini-game.  You start basic and level it up as you uncover new blueprints.  My preferred method of gathering resources has become moving to a new area and building a small base with a scanner room that can pinpoint all the useful spawns for me.  When you disassemble a base you get 100% of the materials back, so the only investment is time.

(Meeple Girl has been playing a very different game.  It involves waiting until I'm under threat by a Warper or a Sea Dragon, then walking quietly up behind and tapping me on the shoulder... it ought to be a crime.)

The last couple of weeks I've been thinking about a method for adding a base-building mini-game to OSR games.  Admittedly, some of them already have it, in the form of castles and strongholds from 9th level onwards.  But that's intended to be a significant investment of time and resources for the characters, and a reward for the players who levelled their characters that far, as well as a completely new mode of play.  I'm going for something a lot simpler and self-contained, that the players can get into almost immediately.  This is what I've come up with.  I think it would work best in a system where XP is earned by spending treasure.

It's assumed that the characters will be taking over the available space across several buildings instead of renting a house or workyard.  They'll modify it to suit their needs, as much as practical given the restrictions.  The spaces they have access to are represented by the seven unique Tetris blocks:

Each square in a piece is 10' by 10'.  The first costs 100 GP, or whatever amount is suitable for your system.  Each additional segment doubles the cost.  The characters can keep building until they have everything they need or run out of money.  Each time the players ask for a segment, roll 1d10 and give them the piece indicated.  If the result is higher than 7, repeat the last block.

After the first piece is placed, roll 1d4 for each additional piece to see which cardinal direction from the original piece the next is added to.  The players can rotate the pieces as they like.

The edges at the outside of the pieces (not the individual squares) are assumed to be walls.  The spaces the players put together have no access to each other until they start knocking out walls.  The edges for each piece must be more than half wall to support its weight.  That's five edges for the O piece, six for the others.  Take out any additional walls and the roof falls in.

Example

My budget for rooms is 1,000gp.

First room = 4, the O piece.  100gp.

Second room = 1, the I piece.  The d4 shows a 1, indicating that it's placed at the top of the O piece.  200 gp.

Third room = 4, the O piece again.  The d4 shows a 2, indicating that it's placed at the right of the first O piece.  400gp.

Fourth room = 2, the J piece.  The d4 shows a 3, indicating that it's placed below the original O piece.  800gp, so no more rooms.

If I remove the walls between the two O-pieces, I get a nice big, open space.  Each O-piece can have one additional wall removed, which gives access to the smaller rooms, like this:

Within the limits above, pieces are placed and rotated at the players' discretion.  So if I preferred I could have had a base like this:

And that second layout looks more useful because of the next rule: courtyards.  You can extend the outermost walls in a straight line.  Where they intersect, they enclose a courtyard.  That long, semi-wide layout gives us a couple of decent courtyards for extra space.

For upper levels, start the pricing again from base.  Use the same layout but with each new floor remove 1d2 of the outermost rooms.  There must be at least one staircase on each floor.

What's the point of all this?  The base is mostly intended to be a fun way for characters to spend more gold, and something for the players to feel invested in.  If you're using the XP-for-spending-gold rule, you're probably also using a carousing table.  The rooms you add to your base will give some minor bonuses for mitigating bad rolls on that.  A forge can knock out the dings in armour, an apothecary can shorten the duration of a hangover, etc and you can hire retainers with the skills to use them on a temporary basis.

So next we slot in rooms.

Apothecary - 1 square.  Gives access to a herbalist.  +1 to saves vs disease, allergies, hangovers, etc.

Armoury - 1 square.  Sharpens blades, repairs weapon handles and straps.

Barracks - requires 1 square for each character that sleeps there.  Restricted to one open edge.  Reduces gold lost during a bad carouse by half because you were sensible and left your big purse at home.

Common room - at least 2 squares.  +1 to retainer morale rolls while in town.

Forge - at least 3 squares.  Must be in a courtyard.  At least 1 empty square between it and the nearest building.  Gives access to a smith, who can repair armour.

Kennel - 1 square per 4 dogs.  Must be in a courtyard.  Gives access to a master of hounds, who can provide training and treat sick dogs.

Kitchen - 2 squares.  Rations prepared here will never be tainted or inedible except by deliberate enemy action.  Requires a well.

Library - at least 1 square.  Gives a bonus to research actions and somewhere for magic-users to work.

Shrine - at least 1 square.  A place for rituals.  Gives clerics and paladins a bonus to communing with their gods.

Stable - 1 square per horse.  Each square must have one open edge facing a courtyard or the outside of the building.  Gives access to an ostler, who can replace shoes and treat sick horses.

Staircase - 1 square.  Gives access to other levels of the building.

Training room - at least 3 squares by 2.  Gives access to a trainer if required for levelling or learning new techniques, subject to GM approval.

Trophy room - 1 square per 4 trophies.  The perfect space for displaying souvenirs of past adventures and impressing young wenches or farmboys.

Wash house - 1 square.  removes fleas, tar and feathers, various other forms of soiling.  Requires a well.

Well - 1 square.  Must be in a courtyard.  At least 1 empty square from a forge.

In this case I'm going to have a training room, an apothecary, an armoury, a wash house, a kitchen, a trophy room, a forge, a well, a kennel for 8 dogs and a stable for 4 horses.  My characters can sleep off site.  Opening up all the stable squares to the courtyard means blocking the stables off from the building interior, but that's not a problem.  If you're riding a horse, you're going outside anyway.

Key:
A - stables
B - forge
C - trophy
D - apothecary
E - armoury
F - training
G - kitchen
H - wash house
I - well
J - kennels







And now the part I'm sure my players will spend the most time over: traps.  200 GP each.

Arrow trap

Collapsing ceiling

Fire trap

Gas trap

Glue trap

Magic item trap

Pit trap (if on an upper floor, requires 1 square of unused space on the floor below it)

Poisoned dart trap

Potion trap

Swinging blade trap

There's no reason to restrict player creativity here.  If they want a false wall that releases angry bees, they can have it.  Any 5' square (4 per map square) can be trapped.  The players can set or deactivate a trap as an automatic action, unless you're a particularly hard-nosed GM.  If characters let a retainer get caught in a trap, make a morale roll at a penalty.

Hey, isn't this starting to look kind of like a dungeon?

Yes!  And for your players, this might be an opportunity for introspection that leads to a revelation about their behaviour and real change on a personal level.  Mine are more likely to say "Hey, why not a trap that douses intruders with shrinking potion then drops them into a trebuchet?"