Boghog's bullet hell shmup 101
The Play Area
This is where all the action happens. The play area can either be contained by the screen, or it can be wider thanks to horizontal panning seen in many vertical shmups. The size of the play area is relative to the size of the game’s objects and hitboxes.
The tiny hitboxes of Danmaku games make for relatively large play areas which allows them to fill the screen with bullets. This has downsides however - large play areas funnel games into relying on lots of projectiles for challenge. It makes it much harder to rely on simple patterns.
The same kind of challenge with different hitboxes, the Danmaku style example has more than twice the bullets.
A game's movement speed and shot type has to be tuned to match the size of the play area, narrow areas can afford to (and can benefit from) slower move speeds and narrow shot types to emphasise small differences in positioning and give enemies time to do their thing (examples : Gunbird 2, Dragon Blaze). Wider play areas tend to use faster speeds, wider shots or some kind of multi directional weapons to compensate for the large distances the player has to travel (examples : Mars Matrix, Under Defeat HD).
The most fundamental source of challenge in danmaku games is identifying, predicting and manipulating different bullet trajectories and making precise movements to dodge bullets and control screen space. Because of this, giving the player as much control, consistency and awareness as you can is the top priority, the movement should feel seamless.
For consistency’s sake, it’s best to avoid different directional movement speeds, such as the ship moving faster on the x axis than the y axis, or faster diagonal movement. Don’t forget to normalise your diagonal movement! There are some great games with faster diagonal movement like Battle Garegga and Armed Police Batrider, but they are rare.
Movement inertia should be avoided entirely because it adds an unnecessary layer of lag to the movement that players have to adjust to, without adding any interesting gameplay dynamics. Even the smallest amount of acceleration will be noticeable to experienced players.
If your movement doesn’t “feel smooth”, then it’s most likely the result of lacklustre visuals rather than a lack of inertia. Some tricks that can make movement feel smoother - beef up your shot/rate, add an afterimage (see Symphony of the Night/Megaman ZX), match your ship’s banking animation speed to movement speed, have the ship leave trails (see Danmaku Unlimited 3), give your ship’s options (floating bits near your ship) some inertia.'
Danmaku games often feature a focus shot mechanic, which was popularised by CAVE. The player’s ship has 2 states with their own speed, a fast wide shot mode (tapping the button), and a slower focus shot mode (holding down the button) usually represented by a laser or another kind of concentrated shot.
This is useful because it gives the player more control and creates some basic but rewarding gameplay dynamics - the players have to think whether they need to use fast movement to quickly get into position, use slow movement to make dodging more precise, or accept slow movement for the sake of additional damage-per-second.
The transition between normal & focused speed is usually (but not always, see Touhou) interpolated to create a smoother, more gradual transition. The speed difference between the normal and focus shot itself varies depending on game & ship (focus being ⅔ of the original speed is a good middle ground).
The gameplay effects of different speeds are highly varied, the usual archetypes you have are fast, powerful ships with narrow shots, or slower weaker ships with wide shots.