Hollowtide level design

Every feature, every level, and the First Night tutorial below it. Hover or tap anything for the detail. Nothing here has been played yet.

LearnNew here, this level teaches it.UseKnown, the game expects it.HelpedReachable, and a helper in the game makes it easier.LockedThe player cannot reach it yet.OffLeft out on purpose at this level.BuiltAlready standing at the start. The player only watches.

	1 Wood	2 Stone	3 Coal and iron	4 Crystal and copper	5 Silver
Hero
Gathering by hand	woodLearn: wood. First moment: the opening seconds, the hero walks to a tree and chops.	stone tooUse: stone too. Stone can be gathered by hand once level 2 opens it.	UseUse. Still the rescue when no extractor reaches a material.	UseUse. Still the rescue.	UseUse. Still the rescue.
Hand-over	wood to towerLearn: wood to tower. First moment: day 1, the hero carries 10 wood to the empty tower: 100 arrows.	UseUse. Carries stone arrows too. Raw stone does nothing at a tower.	UseUse. The rescue for a frozen or empty tower.	UseUse. Expected.	UseUse. Expected.
Pipes
Belts	one beltBuilt: one belt. Stands: one belt from the camp to the tower. Notice: wood riding along it. The player lays none yet.	straight, cornersLearn: straight, corners. Variant: straight, then with corners. First moment: the mine has to reach a tower.	green roadHelped: green road. Helpers: auto-connect (tap the giver, then the taker) and the ghost's green road round obstacles.	longUse: long. Long belts out through the fog are expected.	UseUse. Expected.
Forks and fork rows	OffOff. Open on the bar, but nothing at this level asks for it. No hint points to it.	OffOff. Open on the bar, but nothing at this level asks for it. No hint points to it.	one forkLearn: one fork. Variant: a single fork. First moment: wood must reach both a tower and the arrow maker.	rowLearn: row. Variant: a row of touching forks feeding a row of towers. Helper ready: the fork row drag.	row dragHelped: row drag. Helper: drag once and a whole row lays itself.
Merges	OffOff. Open on the bar, but nothing at this level asks for it. No hint points to it.	OffOff. Open on the bar, but nothing at this level asks for it. No hint points to it.	OffOff. Open on the bar, but nothing at this level asks for it. No hint points to it.	two into oneLearn: two into one. First moment: the kiln's lapis and coal, or a tower with two feeds. The hint says: Ends here, drop a belt merge.	merge hintHelped: merge hint. Helper: when a belt stops at another belt, the hint says: Ends here, drop a belt merge.
Towers
Arrow tower	place oneLearn: place one. One stands already. Notice it shooting. First moment: the other lane has nothing, so the player places one and feeds it by hand.	UseUse. Expected.	UseUse. Expected.	UseUse. Expected.	UseUse. Expected.
Catapult	LockedLocked. Needs core level 2. No stone.	LearnLearn. First moment: night 5, the husk shrugs off arrows.	UseUse. Expected.	UseUse. Works with lightning on the golem.	potsUse: pots. Pots give it fire and ice shots.
Flame	LockedLocked. Needs core level 3. No coal.	LockedLocked. Needs core level 3. No coal.	LearnLearn. First moment: the ice elementals of the first winter.	UseUse. Expected.	UseUse. Also hurts ghosts.
Frost	LockedLocked. Needs core level 4. No lapis.	LockedLocked. Needs core level 4. No lapis.	LockedLocked. Needs core level 4. No lapis.	LearnLearn. First moment: fire elementals on night 9.	UseUse. Expected.
Lightning	LockedLocked. Needs core level 4. No copper.	LockedLocked. Needs core level 4. No copper.	LockedLocked. Needs core level 4. No copper.	LearnLearn. First moment: the golem on night 10.	UseUse. Expected.
Light (holy)	LockedLocked. Needs core level 5. No silver.	LockedLocked. Needs core level 5. No silver.	LockedLocked. Needs core level 5. No silver.	LockedLocked. Needs core level 5. No silver.	LearnLearn. First moment: ghosts through the walls on night 11.
Ammo
Wood arrows	BuiltBuilt. Stands: the prebuilt tower already eats wood from its belt. Notice: arrows flying at night.	UseUse. Expected.	UseUse. Runners still die to them.	UseUse. Expected.	UseUse. Expected.
Stone arrows	LockedLocked. Needs core level 2. No stone.	LearnLearn. First moment: the first stone arrows leave the combiner: 30 shots an item, 4 times the damage of wood.	UseUse. Expected.	UseUse. Expected.	UseUse. Expected.
Iron arrows	LockedLocked. Needs core level 3. No forge.	LockedLocked. Needs core level 3. No forge.	LearnLearn. First moment: husks in number on night 7. A chain of two steps.	UseUse. Expected.	UseUse. Expected.
Fire arrows	LockedLocked. Needs core level 3. No coal.	LockedLocked. Needs core level 3. No coal.	OffOff. Open, but the flame tower does this job here. One new thing.	OffOff. Open, but not asked yet.	LearnLearn. First moment: ghosts, where only fire and light work.
Build
Walls and gates	short wallBuilt: short wall. Stands: a short wooden wall. Notice: runners walk round it.	wall, gateLearn: wall, gate. First moment: the husk breaks through. Shape the road, leave a gate.	UseUse. Guard the ice road.	UseUse. Expected.	UseUse. Ghosts ignore them, which is the lesson.
Reinforce	LockedLocked. Needs core level 2. No stone.	LearnLearn. First moment: a damaged wooden building and stone in the pile.	UseUse. Expected.	UseUse. Fire elementals make it matter.	UseUse. Expected.
Storehouse	OffOff. The core takes everything, so nothing asks for one yet.	OffOff. Same.	OffOff. Same.	LearnLearn. First moment: a far camp, where a local store saves a long belt.	UseUse. Expected.
Combiner, recipes	LockedLocked. Needs core level 2. No stone.	wood + stoneLearn: wood + stone. Variant: one recipe, stone arrows. First moment: raw stone does nothing at a tower, so it goes here first.	more recipesUse: more recipes. Iron and fire arrows, on one tile.	UseUse. Frost crystal too.	hidden recipesHelped: hidden recipes. Helper: recipes above the core level stay hidden.
Extractors
Lumber camp	one campBuilt: one camp. Stands: one camp on a grove. Notice: trees going bare.	your ownLearn: your own. First moment: wood runs short. The circle shows how many trees are in reach.	UseUse. Expected.	UseUse. Expected.	UseUse. Expected.
Mine on stone	LockedLocked. Needs core level 2. No mine.	LearnLearn. First moment: the first level 2 goal.	UseUse. Expected.	UseUse. Expected.	UseUse. Expected.
Mine on ores	LockedLocked. Needs core level 3. Ores stay hidden until their level.	LockedLocked. Needs core level 3. Ores stay hidden until their level.	iron, coalLearn: iron, coal. First moment: the forge needs ore. Ores run out, so the player goes further.	lapis, copperUse: lapis, copper. Further out.	silverUse: silver. Furthest out.
Units and world
Barracks, units	LockedLocked. Needs core level 3. No building.	LockedLocked. Needs core level 3. No building.	OffOff. Open, but not asked yet. Ice and winter are enough.	archersLearn: archers. First moment: a lane the towers do not reach.	UseUse. March with the hero.
Outposts, expeditions	LockedLocked. Needs core level 5. No building.	LockedLocked. Needs core level 5. No building.	LockedLocked. Needs core level 5. No building.	LockedLocked. Needs core level 5. No building.	LearnLearn. First moment: the outpost shows tonight's wave at dusk.
Fog and sight	8 tilesLearn: 8 tiles. First moment: the map beyond 8 tiles is black, and the hero lights it.	10Use: 10. Each level sees further.	12Use: 12. Sees further.	14Use: 14. Sees further.	16Use: 16. Sees further.
Day, night, waves	LearnLearn. First moment: the first dusk.	UseUse. Expected.	UseUse. Ammo changes at dusk.	UseUse. Expected.	UseUse. Expected.
Seasons, winter	OffOff. Summer only, if question 4 is yes.	OffOff. Summer only, if question 4 is yes.	first winterLearn: first winter. First moment: night 6, the lake freezes and a new way in opens.	UseUse. Expected.	UseUse. Expected.
Rifts	two lanesLearn: two lanes. First moment: the first runners, from two sides.	UseUse. Expected.	UseUse. Expected.	UseUse. Expected.	close oneLearn: close one. Variant: kill the guard and the rift closes. First moment: the first expedition.
Core level up	30 woodLearn: 30 wood. First moment: the core's panel shows a bill. Arrows or progress?	two itemsUse: two items. The bill now has two lines.	UseUse. Expected.	UseUse. Expected.	OffOff. Top level, nothing further for now.
Tools
Copy stamp, undo	undoHelped: undo. Helper: undo the last placement, Cmd Z on the Mac, a button on touch. A safe way to try the first tower.	undoHelped: undo. Same helper, for the first own belts.	stampHelped: stamp. Helper: the copy tool stamps a tower and its pipe again. Cmd C on the Mac, a Copy button on touch.	stampHelped: stamp. Helper: stamp whole rows of towers.	stampHelped: stamp. Helper: stamp a working lane on the other side.
Edit mode	OffOff. Too little built to move.	OffOff. Same.	HelpedHelped. Helper: pick a building up and drop it, at no cost, when winter changes the road.	UseUse. Expected.	UseUse. Expected.
New things	7How many things this level teaches for the first time.	8How many things this level teaches for the first time.	5How many things this level teaches for the first time.	6How many things this level teaches for the first time.	4How many things this level teaches for the first time.

The five levels

1Night 1: 4 monsters per rift.2Night 2: 6 monsters per rift.3Night 3: 8 monsters per rift.4Night 4: 9 monsters per rift.5Night 5: 15 monsters per rift.6Night 6: 10 monsters per rift.7Night 7: 14 monsters per rift.8Night 8: 19 monsters per rift.9Night 9: 14 monsters per rift.10Night 10: 24 monsters per rift.11Night 11: 18 monsters per rift.12Night 12: 28 monsters per rift.13Night 13: 39 monsters per rift.

RunnerFast and weak. Arrows stop it.BatFlies over walls and goes for your camps. Arrows stop it.HuskArmoured, smashes walls. Plain arrows do nothing. Catapult or iron arrows stop it.Ice elementalFreezes towers it walks past. Fire stops it.Fire elementalSets wood on fire. Frost stops it best.GolemHuge, breaks a wall in three hits. Catapult and lightning stop it.GhostWalks through walls. Flame and light stop it.Rift motherThe boss. Slow, and spawns runners as she walks.

First Night

5 minutes

Learn the game in 5 minutes, then play.

Move your hero to the trees.Shown: "Move your hero to the trees." Done when: The hero reaches the trees. Then: The next step. Stuck for 20 seconds: "Right click the ground to walk."2

Right click a tree to chop it.Shown: "Right click a tree to chop it." Done when: The hero holds 3 wood. Then: The next step. Stuck for 20 seconds: "Your hero chops one wood a second."3

Hand the wood to the core.Shown: "Hand the wood to the core." Done when: 3 wood reach the core. Then: The next step. Stuck for 20 seconds: "Right click the core. What you bring pays for building."4

Build an arrow tower by the road.Shown: "Build an arrow tower by the road." Done when: A tower stands near the road. Then: The next step. Stuck for 20 seconds: "Pick the tower on the bar, then click the glowing spot."5

Drag a belt from the fork to your tower.Shown: "Drag a belt from the fork to your tower." Done when: Wood reaches the tower by belt. Then: Night falls at once: 2 groups of 3 runners, 12 seconds apart, about 14 tiles from the core. Says: "Night! Your tower and your hero shoot on their own." Stuck for 20 seconds: "Pick the belt, press on the fork, let go on the tower."6

Stop the runners.Shown: "Stop the runners." Done when: Every monster is dead, or the night ends. Then: Dawn comes at once. Says: "Dawn. Wood kept riding the belt, so the tower never ran out." Stuck for 20 seconds: "Walk your hero near them. The bow fires by itself."7

Build a wall across the road.Shown: "Build a wall across the road." Done when: 3 wall edges stand across the road. Then: Night falls at once: 3 groups of 3 runners, 12 seconds apart, about 14 tiles from the core. Says: "More tonight. They will hit your wall." Stuck for 20 seconds: "Pick the wooden wall and drag along the glowing line."8

Hold the road.Shown: "Hold the road." Done when: Every monster is dead, or the night ends. Then: Dawn comes at once. Says: "Dawn. They hurt your wall." Stuck for 20 seconds: "Walk your hero near them. The bow fires by itself."9

Mend the wall with wood.Shown: "Mend the wall with wood." Done when: The wall is back to full health. Then: The next step. Stuck for 20 seconds: "Carry wood to the wall and right click it."10

You are ready.Shown: "You are ready." Done when: The button is pressed. Then: The real game opens on the first map.

3 min 40 s plannedMy estimate of a first-time player, step by step. Not timed with a real player yet.What standsA lumber camp on the grove north east of the core, a belt to a fork, and the belt on from the fork into the core. So wood keeps arriving while the player learns. This is only in the tutorial, not on the real first map.Tutorial rulesThe day waits for the player. A night lasts 45 seconds at most. The core cannot die here, and the hero is back in 3 seconds. The bar shows only what the step needs.Changed from the draftStep 8 split in two, so mending is its own step. The fork stands alone, no fork row. Runners start about 14 tiles away, in groups, so the tower gets a few shots first. Each step only opens the bar buttons it needs.Needs buildingA step runner for these triggers, spawns at a point, and two rules the sim does not have: the core cannot die, and one wall edge loses half its health at dawn, because a runner walks round a short wall by itself.

Wood

sees 8 tilesHow far the core lights the map at this level. A tile is one square, the size of a tower.

Survive 3 nights with wood arrows.

To openstart here

Unlocks

Wood. Your first material. Also your first arrows.

Lumber camp. Cuts the trees within 3 tiles. Belt

Belt. Carries things from what gives to what takes. Fork

Fork. Splits a belt. A row of forks feeds a row of towers. Merge

Merge. Joins belts into one.

Arrow tower. 1 wood is 10 arrows.

Wooden wall. Cheap. Monsters break it or walk round. Gate

Gate. Your people pass, monsters do not.

Storehouse. What arrives is yours to build with.

TeachesWalk, chop, hand wood to a tower, place one tower on the open lane. A camp, a belt and a tower already stand, to watch. Also day and night, and the fog.MixesNothing yet. This is the base for everything after.You win whenThe open lane is held by a tower you placed and fed by hand, nothing hit the core on night 3, and 30 wood reached the core.Why this waySafe first: the first day has no monsters. One new thing at a time. Learn by doing.

Nights, monsters per rift

1Night 1, from each rift: 4 runners.2Night 2, from each rift: 6 runners.3Night 3, from each rift: 8 runners.

New

Runner. New here. Fast and weak. Arrows stop it.

Stone

sees 10 tilesHow far the core lights the map at this level. A tile is one square, the size of a tower.

Dig stone, stop the husk.

To open30

Wood. Delivered to the core, by hand or belt.

Unlocks

Stone. Arrives 3 times slower than wood. Mine

Mine. Digs the rocks within 3 tiles.

Combiner. Three belts in. What arrives decides what it makes. First recipe: wood and stone.Stone arrows. 3 wood and 1 stone make 1 stone arrow item: 30 shots, each 4 times a wood arrow.Reinforce. Pay stone to turn a wooden building to stone. Double health.

Stone wall. Strong. Mend it with stone.

Catapult. Throws big rocks far. Feed it stone.

TeachesYour first own belt, with corners. Your own camp. The mine and its circle of rocks. The combiner: wood and stone make stone arrows. The wrong ammo does nothing. Reinforce. Monsters that break walls.MixesThe wood belt now goes through the combiner, and one belt of good arrows feeds the towers. Walls shape the road to the towers.You win whenA catapult kills the husk, the camp a bat attacked still stands or was rebuilt, and 100 wood and 30 stone reached the core.Why this wayFair answer: the catapult opens before the husk arrives. Rise, rest, rise: night 4 is a breather.

Nights, monsters per rift

4Night 4, from each rift: 8 runners, 1 bat.5Night 5, from each rift: 12 runners, 2 bats, 1 husk.

New

Bat. New here. Flies over walls and goes for your camps. Arrows stop it. Husk

Husk. New here. Armoured, smashes walls. Plain arrows do nothing. Catapult or iron arrows stop it.

Coal and iron

sees 12 tilesHow far the core lights the map at this level. A tile is one square, the size of a tower.

Make iron arrows, burn the ice.

To open100 Wood

Wood. Delivered to the core, by hand or belt.30 Stone

Stone. Delivered to the core, by hand or belt.

Unlocks

Coal. Feeds the flame tower.

Iron ore. Dug by a mine. Goes to the forge. Iron

Iron. Made from iron ore.

Iron arrows. Go through a husk's armour.

Fire arrows. Burn. Good against ice and ghosts.

Iron forge. Iron ore in, iron out.

Arrow maker. Wood and iron in, iron arrows out.

Flame tower. Burns everything close to it.

Barracks. Makes archers and knights from a belt.

TeachesOne fork. A chain of two steps: ore to iron, iron to arrows. Flame. Winter, when the frozen lake opens a new way in. Changing a tower's ammo at dusk.MixesThe fork splits wood between towers and the arrow maker. The hero's hand-over rescues a frozen tower.You win whenIron arrows reach a tower by belt, the ice road is guarded before it opens, and the core survives the boss.Why this wayOld tool, new use: the fork and merge do a new job. Mixing what you know makes it harder, not new rules.

Nights, monsters per rift

6Night 6, from each rift: 8 runners, 2 ice elementals.7Night 7, from each rift: 10 runners, 2 ice elementals, 2 husks.8Night 8, from each rift: 12 runners, 3 ice elementals, 3 husks, 1 rift mother.

New

Ice elemental. New here. Freezes towers it walks past. Fire stops it.

Rift mother. New here. The boss. Slow, and spawns runners as she walks.

Crystal and copper

sees 14 tilesHow far the core lights the map at this level. A tile is one square, the size of a tower.

Reach far, slow the crowd.

To open300 Wood

Wood. Delivered to the core, by hand or belt.100 Stone

Stone. Delivered to the core, by hand or belt.30 Iron

Iron. Delivered to the core, by hand or belt.

Unlocks

Lapis. A blue mineral. Slows monsters in a frost tower. Copper

Copper. Feeds the lightning tower.

Frost crystal. Made in the kiln. Freezes monsters.

Crystal kiln. Lapis and coal in, frost crystal out.

Frost tower. Slows and freezes.

Lightning tower. Lightning jumps between monsters.

TeachesFork rows, merges and units. Slowing monsters. Lightning that jumps.MixesLong belts out through the fog. The kiln needs new lapis and old coal. Slowing helps the catapult land its slow shots. Reinforce protects wooden lines from fire.You win whenA long belt survives the night, a golem falls to catapult and lightning together, and the wood for the next bill is on its way.Why this wayFair answer: frost opens before fire elementals come. Open room: the player picks where the long belt runs.

Nights, monsters per rift

9Night 9, from each rift: 10 runners, 2 fire elementals, 2 bats.10Night 10, from each rift: 15 runners, 3 fire elementals, 3 husks, 2 bats, 1 golem.

New

Fire elemental. New here. Sets wood on fire. Frost stops it best. Golem

Golem. New here. Huge, breaks a wall in three hits. Catapult and lightning stop it.

Silver

sees 16 tilesHow far the core lights the map at this level. A tile is one square, the size of a tower.

See what comes, close a rift.

To open1000 Wood

Wood. Delivered to the core, by hand or belt.300 Stone

Stone. Delivered to the core, by hand or belt.100 Iron

Iron. Delivered to the core, by hand or belt.30

Frost crystal. Delivered to the core, by hand or belt.

Unlocks

Silver. Far from home. Feeds the light tower.

Light tower. Hurts ghosts and mends walls near it.

Pot maker. Makes fire pots and ice pots for the catapult.

Fire pot. A burning catapult shot. Ice pot

Ice pot. A freezing catapult shot. Outpost

Outpost. Shows at dusk what a nearby rift will send.

Expedition. The hero leads units to a rift and kills its guard. That rift closes for good.

TeachesReading tonight's wave from an outpost. Ghosts that walk through walls, and fire arrows against them. Pots. Marching units to a rift.MixesEverything. Ghosts undo walls, so towers must cover them. Pots feed the catapult. Units march with the hero. The player picks the mix for each rift.You win whenThe player names tonight's threat at dusk and changes a belt in time, and one rift is closed, or the core stands after night 13.Why this wayShow, test, mix: nothing is taught alone here any more. One clear goal over a big open choice.

Nights, monsters per rift

11Night 11, from each rift: 12 runners, 2 ghosts, 2 ice elementals, 2 fire elementals.12Night 12, from each rift: 16 runners, 4 ghosts, 3 ice elementals, 3 husks, 2 golems.13Night 13, from each rift: 20 runners, 6 ice elementals, 5 husks, 2 ghosts, 2 golems, 3 bats, 1 rift mother.

New

Ghost. New here. Walks through walls. Flame and light stop it.

Seven questions for Yassine

Green is my pick. Hover or tap a card for what yes and no change.

1One long run?YesNoAll 5 levels in 13 nights, about an hour.13 nights times 4.5 minutes is about 58 minutes, and today's run is 6 nights. If yes: one map, one base, growing all the way. If no: each level is shorter or starts from a saved base, and we redo the night plan.2Monsters wait for the core?YesNoA new monster comes only once its answer is open.Today monsters follow the night count, so a player who grows late meets level 3 monsters at level 2. If yes: a new kind waits for the core level, and only the count follows the night. If no: growing late is punished hard.3Move fire and ghosts later?YesNoFire elementals to night 9, ghosts to night 11.Today fire elementals come on night 2, but frost, their weakness, opens at level 4. Arrows still hurt them, so it is not impossible, only unfair. Ghosts come on night 6 today, which fits flame at level 3, but holding them keeps one new thing per level. This is a change to the waves file.4First winter on night 6?YesNoWinter starts with level 3, not inside level 2.The rule of 3 nights each season puts winter on night 4, in the middle of learning stone. If yes: the 3 and 3 rhythm changes for the first summer. If no: level 2 teaches stone and winter at once, against the one new thing rule.5Keep the big bills?YesNoKeep them for now, then test in a real play.Level 5 costs 1000 wood. One camp makes 30 wood a minute, so 3 camps need 11 minutes, about 2.5 rounds, with nothing spent on arrows. One mine needs 30 minutes for 300 stone. The bills for levels 4 and 5 are a continuation of Yassine's pattern, still to confirm.6Win by holding on too?YesNoClose a rift, or hold through night 13.The design says closing every rift wins. If yes: a player who holds through night 13 also wins, so a player who never mounts an expedition still gets an ending. If no: closing the rifts is the only win, and the run has no fixed length.7Start level 1 with things built?YesNoA camp, a belt, a tower and a short wall stand on the first map.Proposed on the first map: a lumber camp on the grove north east of the core, at (4.5, 6.5), a 3 tile belt south to an arrow tower at (4.5, 2.5), and a 3 edge wooden wall by the east road. The player only walks, gathers and explores, and watches wood ride the belt and the tower shoot. The west lane stays open, so they still place one tower and feed it by hand. If yes: I add these to first.json. If no: the player builds all of it, and level 2 is lighter. Nothing changes in first.json before a yes.

Names for the game

1HoldfireMy pickKeep the fire, and hold your fire until they come.Two meanings in one word: the fire you protect, and the order a defender gives. No game of this name found. I searched the web for a game of this name, which is not a legal check.2HollowtideOptionA hollow in the mountains, and monsters coming in like a tide.The working name, and it fits the new valley map. No game of this name found. I searched the web for a game of this name, which is not a legal check.3RiftwardOptionSays the job: ward off the rift.Clear but plain. No game of this name found, though RiftGuard is close and on Steam. I searched the web for a game of this name, which is not a legal check.

Names for the core

1HearthfireMy pickA fire you feed with wood. The word the game already uses.No renaming work. Two cautions: the core's model is now a stone and a crystal, not a fire, and Skyrim has an add-on called Hearthfire.2KeystoneOptionThe stone that holds the arch: lose it and all falls.Matches the stone and crystal model and the way you lose. Feeding a stone with wood makes less sense.3WardstoneOptionA stone that keeps monsters out.Matches the model. Sounds like many fantasy games, and it keeps nothing out until you build.

What the game has today

today: Day and night, fog, hero gathering and hand-over, arrow tower, flame, frost, lightning and light towers, catapult, Reinforce, combiner and refiners, rifts and waves, seasons.
being built: Everything on the one tile grid, belts, forks, merges, walls and gates. Core levels are in the sim, and the screen is catching up.
soon: Barracks and units, outposts and expeditions. They have rules and numbers, and have not been played in a level plan.

Sources, and what each taught us

Mostly blogs, wikis and player guides, not studies. I found no real source for the four-step lesson being Nintendo's, so nothing here claims it.

Cornell level design lecture: a level does not need a new mechanic each time.
Bugnet: levels that teach without telling: one thing at a time, then combine (with the next two).
Bugnet: tutorial levels: same lesson.
Wayline: tutorials as level design: same lesson.
Stepico on challenge: challenge just above skill, low penalty early (with the next two).
Game Developer: pacing in Puzzledorf: same lesson.
Wayline on onboarding: same lesson.
Kingdom Rush level by level: one new enemy can carry a level.
Factorio Friday Facts 241: too tight a tutorial teaches the tutorial, not the idea.
Satisfactory Tier 0: a fixed opening, then freedom.
Satisfactory Milestones: later steps open by delivering goods, like our bill.
Mindustry player discussion: each sector opens the next and a new material.
Mindustry Build 150 notes: the order took years of tuning.
Kingdom Two Crowns: stone: stone on island 2, iron later.
Kingdom Two Crowns: islands: a ladder tied to a place, not a level.
Stormgate on onboarding: few strategy campaigns pace new things well.