A bullet hell fills the screen with enemy fire arranged into patterns and gives you a hitbox a few pixels wide. Shooting is the easy half — you hold the button. The game is reading: naming the pattern that has arrived and being in its gap when it closes. Bullets come in kinds, and each kind teaches a different skill.
Your ship is not its sprite. The part that can die is a dot a few pixels across, often drawn for you in a contrasting colour, and the rest is decoration the bullets pass through. That dot is the fairness. Sparse, fast bullets kill by surprise; dense, slow bullets kill by your own mistakes, so shrinking the target and flooding the screen with fire you can see coming makes a game harder and fairer in one stroke. It also means you move in taps, not sweeps — a big movement walks you out of one gap and into somebody else's pattern.
Japanese arcade studios worked this out in the early nineties. Toaplan's Batsugun is the usual proto-danmaku citation, and Cave — founded by Toaplan staff after it folded — made the style a genre with DonPachi. Danmaku means barrage, or bullet curtain, and the word is the whole design brief.
A shot, or a tight cluster, launched along the line between the emitter and you at the moment it spawns. Move and the next one comes to your new position, so the pattern ends up a portrait of your own recent movement.
Stand still and you die. Drift steadily in one direction and the shots string out behind you in a diagonal rope — streaming, and the first thing that separates a player from a victim.
Several shots leave together at fixed angles, so the gaps between them widen as they travel. Up close it is a solid wall. Far away it is a fence with room between the posts.
Closing on the source is exactly wrong, and so is retreating straight down the middle. Walk out of the side of the cone — you only need to stop being in front of it.
A rank of bullets sweeps toward you with a deliberate hole in it, often shifted a notch per wave so the seam walks across the screen.
Find the seam while the wall is far away and set off early. The version that kills people is the one where the gap was reachable and they spent the trip confirming it.
The source turns while it fires, so the bullets curve outward in arms. It looks static from a distance and it isn't: every gap rotates, and standing in one means waiting for it to sweep over you.
Move against the rotation and the arms open ahead of you. Move with it and you ride a closing gap into the wall.
A slow field laid over a fast one — a drifting grid with aimed shots threaded through it, say. Gaps exist only where the holes in both patterns coincide, and only for an instant.
You cannot read both. Treat the slow pattern as the floor plan and the fast one as timing: choose a route through the slow layer, then wait for the fast layer to let you take it.
Three words carry most of the genre's meaning. A graze is a near miss counted on purpose — several games pay score for passing close to a bullet, which turns fear into greed. A bomb is the panic button: it clears the screen at a price, and dying with bombs in reserve is the classic beginner's signature. And the 1cc, the one-credit clear, is the goal that matters, because continues let anyone credit-feed their way to the ending. Finishing isn't the achievement; finishing on one coin is.
A full run is twenty-odd minutes — not a short game badly padded, but a game whose length is counted in attempts. Above the 1cc sit the scoring systems, the second loops and the true final bosses.
This one preaches to the converted and knows it. From outside the screen looks impossible, so difficulty does the marketing, and twenty-odd minutes of content looks like poor value to anyone who won't replay it — while the depth lives precisely in the replaying. Arcade convention goes unexplained in-game, too, because the assumed player already knows what a bomb is for.
The patterns travel better than the genre does. Ikaruga made them a routing problem by letting you flip polarity and absorb same-colour fire; Touhou grew a doujin culture around bosses whose barrages read as personalities. Loosen them into improvised crowds and you have twin-stick shooters; fold them into procedural floors and you have the roguelites that carried danmaku to a far wider audience. The mirror image is the survivors-like, nicknamed bullet heaven with a straight face, because there you are the one emitting the wall.
Type: Bullet hell (danmaku — a shmup where the pattern, not the enemy, is the obstacle)
Core loop: Name the pattern, find its gap, thread it in small taps, survive the boss's phases
Perspective: Top-down, fixed vertical or horizontal scroll, no camera to manage
Win / lose: Reach the final boss and beat it; one hit takes a life, and lives are few
Session: A run is twenty-odd minutes; mastery is counted in runs, not hours
Progression: Nothing banked — score, patterns learned, and the shot type you chose
Design patterns, not difficulty. A wall of bullets with a findable seam is the most generous obstacle in action games, because it shows you the answer before it asks the question. Take the seam away and you have not built a harder bullet hell. You have built a screen that kills at random and charges a credit for it.