Souls Aboard: How Your City Grows

Births, the two ceilings that cap you, module slots, and the late-game immigration lever — the complete growth model, in plain language.

Births

Your breeding population is free ADULT citizens plus assimilating workers — their children are born free. Children of your own are excluded: the 5% rate below is a fraction of the adults who can actually start families, not of everyone aboard. Slaves are excluded entirely; they can't start families under your flag. Each tide, growth moves a fraction of the gap between your current population and your capacity:

• Family policy (Govern) is a real lever: Encourage gives +60% births and +1.0 happiness, paid for with a childcare subsidy of $1.10/tide per citizen. Restrain cuts births by 55% (to ease crowding or supply) but costs −2.5 happiness — citizens resent being told not to have kids.
• The development taper is the demographic-transition curve — big, advanced, prosperous flotillas have fewer native births, mirroring the real-world pattern. It's a composite score saturating on population (half-strength at 200,000 souls), technology (half-strength at tech 800), and happiness (half-strength at 20), and it floors at 30% of the base rate — native growth never stops outright, it just slows a lot. Immigration is deliberately exempt from this taper, which is what makes it the designed late-game growth lever once native growth has tapered (see Immigration below).

Who counts as a child

A child is anyone who joined your citizen count within the last 5 tides. They are real souls — they eat, they drink and they need housing like anyone else — but they pay no levy, can't be recruited, and don't turn out for the militia. Five tides on they graduate to adults on their own. There is no per-person age anywhere in the simulation, only that rolling window, so the census figure is a cohort, not a birthday.

That window counts each tide's gains — a tide that lost souls contributes nothing, and it never cancels another tide's arrivals (a hard week of departures followed by a welcomed convoy still shows the convoy's dependents as children). Arrivals who row over as grown adults are not counted as newborns either: every inflow is split into a child share and an adult share the moment it lands, and only the child share sits in the window. A founding wave and an open-borders tide each draw a variable share — roughly 5–30% children, so most of any boatload is grown and goes straight onto the tax base and the militia roll. A refugee convoy has its own split: half join the workforce directly, and the rest land in the citizen count as dependents.

Who is off the rolls while ill

Sickness takes a soul out of the working population for as long as it lasts. The citizens and workers your infirmary counts as needing a bed pay no levy and turn out for no militia that tide — and that holds whether or not a bed was actually free for them, so skimping on beds never keeps a patient on the tax rolls. They are still aboard, still eating and drinking, and still your responsibility: the ones left without a bed are the ones who start dying.

Once a tide, your Chief Physician files a report on how the sick bay is doing — how many souls are on the sick list, how many she lost, whether the wards are filling up or freeing up, and how many new souls were born aboard. When there's real news it lands as a popup from her; the same figures also sit on a Doctor's Report card on the Medical screen, so you can check in on the infirmary without waiting for the next report.

The two ceilings, part one: housing

Your population capacity is a flat command-deck floor plus what your infrastructure supports:

• The 100-soul command-deck floor is unconditional — a flotilla stripped of every last point of infrastructure still shelters its founding hundred.
• Every infrastructure point supports 6 citizens at neutral happiness, and happiness swings that number ±4% per point — a genuinely happy flotilla houses noticeably more per infra point than a miserable one, floored at 25% of the base rate so misery can never zero your capacity outright.
• The housing bonus is the multiplier that actually changes the shape of your growth curve: Habitat Decks (+8% capacity each, up to 5 copies), Arcology (+12% each, up to 2), and Leviathan Dock (+15%, 1 copy) all stack. They're also the three biggest food and water producers in the improvement catalog — Arcology alone adds 9,000 food and 7,000 water per copy.

The two ceilings, part two: provisions

Every soul aboard eats one unit of food and drinks one unit of water each tide, at full rations. Production comes from your open deck (a population-capped subsistence catch plus a small uncapped surplus per platform section), built production buildings, and wonders — all scaled up by technology. Whichever of housing or provisions binds first is your real ceiling — a flotilla with room to house 50,000 souls but food for only 30,000 still stalls at 30,000.

Overshoot either ceiling and nothing happens immediately: citizens tolerate crowding for a 2-tide grace period, then 30% of the excess emigrates each tide after that until you're back under capacity. Rationing (full / reduced / emergency) buys headroom at a real happiness cost, and pushing captives below a starvation threshold on emergency rations kills them slowly instead of your citizens.

Module slots: what gates the catalog

Every non-starter improvement type draws from one shared slot pool: you unlock a new module slot every 1,000 souls aboard (citizens, workers, slaves, and prisoners all count toward that total). Once you have a free slot, you can build any improvement you can afford in any order — there's no separate per-building population gate anymore, only the shared slot budget, each type's own copy cap, and the occasional prerequisite (a Drydock needs a Harbor, a Mint needs a Bank, and so on). Drift Nets and Rain Catchers are exempt — unlimited, free upkeep, no slot cost — which is why "build food and water first" is always safe advice regardless of where your slot budget stands.

Immigration: the late-game lever

Open Borders (Govern → Immigration) adds a small daily inflow scaled to your free housing capacity, at a real happiness cost (−1.5). Because immigration sits outside the development taper described above, it's the lever that keeps working after your native birth rate has slowed under a big, advanced, happy population — exactly the flotilla shape where native growth taper bites hardest. Closed borders turns most incoming refugee-convoy world events away before they're ever offered to you. Each tide's inflow arrives as a mix of adults and children (see Who counts as a child); the grown majority is working and taxable the same tide it steps aboard.

Arrivals, assimilation, emigration & the floor

• Founding Arrivals. Every new flotilla takes aboard exactly 200 souls across 5–9 waves, landing on random days 1–14 — the same total for every account, differently timed, so a cohort's founding month is fair by construction. Each wave arrives mostly grown (see Who counts as a child), so a founding flotilla's census will show a fair few children early on — those are its own births, not the drifters.
• Bought workers assimilate into full citizens on a normal timer, tracked per cohort.
• The population floor is two-layered. No flotilla, ever, falls below 100 souls — a hard floor. Above that, population can't fall below 50% of its own windowed peak (the last 30 tides) — a proportional floor that follows a growing flotilla up as it grows.
• Sell-back eviction. Selling infrastructure or platform back to the market for cash can unhouse citizens; if it does, they depart in full on the very next tide, not gradually. Cut housing with your eyes open.

The classic trap