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 citizens plus assimilating workers — their children are born free. 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 is measured against your citizen total, which means arrivals who row over as grown adults would otherwise be counted as newborns. They aren't: 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.

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