The Skills screen is the whole of your progression, and it opens the game up before it tunes it. The four skills touching the Core are features — Caskets, Crafting, the Dealer and Building — and they are the cheapest things on the tree, with Shields, the Bounty Board, Mods, Siege and Farms a step further out. Stat bonuses like more health or faster reloads branch off behind whichever feature they belong to, so you unlock what the game is first and sharpen it afterwards. Features cost Certs and something you have actually done, like a number of kills or a round reached, so the tree tells you what to go and do next rather than just what to save up for.

You are a paperclip. Everything you shoot runs on paper ammo. Paper isn't handed to you — shoot the paper deposits (stacks of reams and shredder bins) that keep appearing around the map and they'll burst into ammo. Watch the minimap for the pale square blips.

Pressing Play drops you into a randomly chosen arena: the Colosseum, a Ghost Town, an open Field, the Suburbs, or the Paper Mill. Houses and sheds can be walked into — the roof lifts off while you're inside. Burning dumpsters are live explosives: shoot one as the horde walks past and it takes the lane with it. Zombies come in rounds — clear every zombie in the round to end it, then take a short breather before the next one. They are not all the same: Poison zombies leave you bleeding damage over time (drink an Antipoison Stew to clear it and shrug off the next one), Ice zombies freeze you slow, Ember zombies burst into fire when they die, Static zombies jam your reload, Runners sprint, Brutes soak damage, and rare Alphas hit like a truck.

Shields are earned, never bought — craft one at the bench or find it in a casket. They are not a ladder: the Bulwark Plate takes a third less damage but slows you down, Quickfold halves your reload and costs you health, Thorn Fold sends half of every hit back, Siphon Ward wrings paper out of the bodies, Ember Ward lets you stand in your own fires while they burn twice as hard, and Second Skin only wakes up once you are nearly dead.

Every so often an ordinary zombie turns up as a Champion — same variant, same shape, but three and a half times the health, a heavier hit and a cold blue glow. Only one is ever out at a time and never before round three. It is worth the trouble: a Champion is carrying a locked chest, and killing it drops one on the spot even if the map is already holding as many as it allows.

Locked chests turn up on the map now and then — hollow diamonds on the minimap — and hold one or two things genuinely worth having: a weapon or shield you do not own yet, mod parts, a stack of good materials, a pile of caskets. You need a Lockpick to get in. Zombies drop them and the Dealer carries them.

Picking one is a pin tumbler. Every pin has a height you cannot see; push each one where you think it sits and turn the wrench, and the lock tells you per pin whether you were too high or too low. Pins that find the shear line stay seated. You get three turns — and that is exactly enough, because halving the remaining range each time cracks any lock in the game every single time. Guess wildly instead and the pick snaps. Arrow keys or the mouse set the pins, Enter turns, and backing out before your first turn costs you nothing at all.

Caskets are the gold diamonds on the minimap, and zombies drop them too. Picking one up never interrupts a fight — it goes straight onto your pile. Open them from the Caskets screen on the main menu, one at a time for the full reveal or the whole pile at once. A casket picked up deep into a run is worth more than one from round two, and it keeps that value however long it sits there.

Back in the main menu, the Crafting screen turns those materials into guns, knives, grenades, molotovs and shields. Crafted weapons and shields are permanent; throwables are stock you carry into a run and whatever survives comes back with you. Pick up to three weapons in the Loadout tab — that's what you spawn with.

The Bounty Board stands a few paces from where you spawn. It does not wander, so you always know where the work is. Everything on it is combat — kill a count of a particular variant, land a run of headshots, reach a combo, put down a Champion, an Alpha or a Landfill, clear a round without being touched, do a stretch of killing with one class of weapon — and everything it pays is combat too: paper, throwables, plating, a mod part, sometimes a casket or a lockpick. You can carry two jobs at once, and each one finished back to back pays up to 60% more until you abandon one and break the streak. The postings refresh every round.

The Dealer starts where you spawn and then carries his stock around the map on his back. He sells three items at a time — weapons, food, throwables, crafting materials, paper, wood — for Certs — the same ones the skill tree wants, so every purchase here is a skill you are not buying. His stock restocks itself every minute, or you can pay to reroll it early (the reroll price climbs each time until the free restock resets it). He also buys: the Sell tab takes crafting materials and spare mod parts off your hands at a fifth of what he charges, which is the only way to turn a pile of coal you will never use back into Certs. Materials are saved to your profile, so anything you sell is gone for good. There are no banks. Food supply drops also parachute in at random and restock your healing items. Wooden crates and shacks are still full of Wood — shoot them down and press B to build.

Training on the main menu opens a hall where nothing counts. A line of dummies reports the damage and DPS you are actually doing, headshots included, so you can feel out a gun or a mod combination without spending a run on it. A cage off to one side takes any enemy in the game at any round's strength — pick the type, the count and the round, then open the gate if you want them to reach you. Racks along the walls hold one of every weapon and item there is, and paper is unlimited. Certs, materials, caskets, achievements and the bestiary are all switched off while you are in there, which is exactly why the racks can be that generous.

From round three on, the breather between rounds lays out three rules for the round you are about to fight, tamest on the left, with what each one pays stacked next to it. Click one. A rule that doubles the wave, blinds you, or blunts a whole class of weapon pays up to two and a half times as much — on the round-clear reward and on the drops while you are in it. The plain round pays exactly what it always did. The first two rounds are always plain, and rules ease in over rounds three to five rather than landing at full strength.

There is a ladder leaning against the edge of every map, and it is the only way a run ends that is not dying. It stays sealed until you clear round ten, then lights up and stays open for good. Stand at the foot of it and hold E for four seconds — take a hit in that time and you are knocked off and have to start the climb again.

Getting out pays an extraction bonus that grows sharply with the round you reached and the work you did getting there, plus a armful of caskets. Dying pays none of it. Everything else you earn is banked the moment you earn it, so the ladder is the one thing in the game you can actually lose by pushing one round further.

Press G to aim a throw. A ring follows your cursor showing exactly where the thing will land and exactly how much ground it will cover — that ring is the real radius, not a decoration — with the flight path dotted out in front of you. It turns red where the throw cannot land. Click to let go, G or the mouse wheel swaps between the Java Bombs, Frags, Glue and Spikes you are carrying, and right click or Esc backs out without spending anything. Throwables used from a weapon slot land on the same spot, so the cursor always decides.

Press B for build mode. A translucent wall follows your aim, snapped to the nearest grid edge — green means it will build, blue means it will edit what's already there, red means it can't. Click to place. Because pieces sit on grid edges they line up exactly, so you can run a wall along a street or box off a doorway.

Click a wall you've already built and it cycles through three forms. A Solid wall stops everything, including your own shots. A Window is waist-high: bodies are stopped but you fire straight over it. A Doorway lets you walk through while the horde cannot. The extra wood is only charged when you upgrade into the fancier form.

Weapons have weight. A pistol or a blade costs you nothing, but a drum-fed Shredder slows you by nearly a fifth and an Origami Dragon by almost a third. Only the weapon in your hands counts, so swapping to a sidearm to reposition is a real move, and a heavy gun is something you commit to.

Weapons in the same class are not ranked against each other. The Snub Roller hits far harder than the Clip Pistol but only holds six; the Ash Sprayer barely scratches anything yet sets the ground alight; the Quill Lance trades damage for punching through four bodies at once; the Frost Duster leaves everything it touches crawling. Pick for the round rule and the map, not for the biggest number.

Aim at a head and the reticle narrows and turns red; hit one and the shot does double damage. Aim assist points at the chest unless the cursor is genuinely on the head, so a head shot is something you line up on purpose — and spread works against you, which is why a Quill Sniper takes heads across the map and a Shredder mostly doesn't.

Mods are parts you craft from materials and bolt into a weapon — two slots each, and the parts fit any weapon you own, forever. None of them is simply better: a Bored Barrel halves your spread and slows the gun down, a Twin Nib throws an extra projectile that hits softer, Lamination takes half the weight penalty off a heavy gun and blunts it, and a Chaos Sight makes heads worth triple while everything else lands for less. Fitting a part uses it up, and pulling it back out destroys it, so a build is something you commit to rather than shuffle between guns.

Weapons come in five rarities — Common, Uncommon, Rare, Epic and Legendary. Every one can be crafted if you gather the materials, but caskets also turn up weapons you don't own yet, and the rarer the weapon the longer you'll wait. Anything you find that way is yours permanently, exactly like a crafted one.

Siege engines are built from kits you craft in the Siege tab. Select a kit, click to set it down in front of you, then stand next to it and press E to pour in paper from your own reserve. They pick their own targets and hold a lane while you're somewhere else — but every sheet you give one is a sheet you can't fire yourself, and you can only have three standing at a time. The Ream Turret chatters at anything close, the Confetti Cannon throws a wide cone, and the Quill Ballista reaches most of the way across the map.

Farm machines are the other half of the Siege tab, and the most expensive things you can build. They don't shoot — they bait. Set one down and any zombie that is closer to the machine than it is to you stops caring about you and walks to the slab out front instead. The machine takes it, burns it, and fires its loot out of the chute on the far side. No loading, no ammo — proximity is the whole trigger. Put one at the end of a corridor you've walled off and it will work a lane on its own while you're somewhere else.

Reprinting is what you do when there is nothing left to unlock. It costs Jugs of Wine — a common material you would otherwise burn on Java Bombs, so reprinting means hoarding something useful on purpose. It resets your skill tree, your Certs and your unopened caskets, and it keeps everything that took real time: every weapon and shield you crafted, all your other materials, your rank and everything it unlocked. In exchange, drops widen by 15% permanently and materials and caskets get 25% more likely, every time. The wine cost climbs with each edition.

Origami cats wander every map and want nothing to do with the fighting. Shoot one and it comes for you, and however it dies it comes apart into rolling paper. Use a piece of Gold next to one instead — zombies drop it — and it is yours. A tamed cat follows you, and it fights: not the horde in general, but whatever has just hurt you. Zombies always want you first and only turn on the cat when they have genuinely lost sight of you.

A cat has twenty health and does not heal on its own. Stand almost on top of it and press Z to hand over food meant for you — the range is deliberately tiny so you never do it by accident. Press E on it to put it in your bag, where Z sets its job and X puts it back down. Heel keeps it close, Fetch sends it after loose drops, Sniff has it hunt down caskets and paper and dig up more of both, and Carry lets you hold one more kind of item. A cat in your bag when a run ends is still there at the start of the next one; a cat left out on the floor is not.

When a cat runs out of health it does not vanish — it lies where it fell. Find a Bandage off a zombie, stand over the body and hold Z for ten unbroken seconds, and it gets back up.

From round two onward a growing share of each wave arrives as Skeletons, levelling off at about a quarter. They do not close on you — they hold a gap of about fifteen paces, sidestep, and shoot. Backing off is exactly what they want, so the answer is to push in: they die in far fewer hits than a zombie once you reach them. A crowd of zombies pushing you back into a line of archers is the fight this game is really about.

A Ghoul can turn up at any point in a round — usually once, sometimes not at all, rarely twice, never more. It is faint enough to lose against a busy floor and faster than anything else in the game, with no name tag or health bar to give it away. It is glass, though: land a hit and it goes down in far fewer shots than a zombie. The difficulty is spotting and tracking it, not surviving it.

Every fifth round, a boss takes most of that round's slot — the ordinary wave is thinner while it's out. It's slow enough to create space from, but it hits far harder than anything else in the game and gets stronger with every boss round that passes. It has to die for the round to end. Check the Bestiary on the main menu for anything you've faced before.

Things you leave on the ground react with each other. Order matters — put the second thing on top of the first.

Fire is fire wherever it came from — a Java Bomb, a Tinderbox ring, or an Ember zombie bursting on its own. That last one means the horde can set these off without you.

The shape tells you what kind of thing it is, so you never have to memorise the colours. Anything round is alive and wants you dead.