[Trapdoor]
Trapdoor animation lasts about 2.1 secs, which is equivalent to ~5 beats. Sometimes it's faster to take farther stairs instead.
If you push crate/barrel down to trapdoors, their content will appear on your left in the start room of the next floor. If it contained an enemy, it will turn into golds.
[Spike Trap]
If you have lead boots, you can leave enemies behind and go through trap room (especially in Z4, with map/torch of foresight).
Even without lead boots, sometimes you can choose your route wisely to kill enemies behind with spiketrap.
You can push armored enemies (ex. blademasters) to spiketrap for faster kill.
[Bomb Trap]
On Eli, you can bomb-kick bombs from bomb traps.
[Scatter Trap]
You can use scattertrap to remove your nazar charm, which can be useful on Aria score. (Even some of WR tier run doesn't remove nazar. It's theoretically good, but can ruin your run too.)
You can use scattertrap to remove your torch or grenade/bomb charm, which can make duping score easier.
You can use scattertrap to place unnecessary items on the floor, and transmute them.
If you use scattertrap and recollect anything except your dagger, it breaks low%.
Scattertrap scatter your items in this priority (get blocked by enemies, golds, and walls).
[Tempo Up/Down Trap]
Tempo-up trap worth only 2 beats. You don't need to drift from your path to activate it.
[Tiles]
Earth deletes water, tar, hotcoal, ice, goo, wire.
Freeze deletes hotcoal and turn water/tar into ice.
You can use spells/items while sliding, which can be used to prevent damages. (ex. get iframe with food/heal, use freeze/shield, or use earth to stop sliding.)
[Stairs]
Earth spell doesn't work for stairs. You can't earth-kill an enemy on it.
As used above, you can use blood heal without blood magic cost on stairs. (Normally once on single-tempo characters, twice on Bolt, a lot on Bard.)
And you can do it twice (3 times on Bolt) if it's Death Metal song. (See here)
With iframe, you can take trapdoor on Aria/Coda without losing potion nor dying from missbeat. However, most of them requires subbeat-timing.
Taking a trapdoor with potion on Aria depth-5 is a good option too.
Timing (Aria):
Potion: Any
Food: Late to trappdoor -> Early food (I've also heard early -> early is more consitent, idk)
Courage Shovel + Earth: Early
Courage Ring: Early
Blast Helm + Leaping/Lunging: Early
Shopbro: Any (on the same turn as you step into the trapdoor)
Tank on trapdoor: Any
Tank before trapdoor: Early
Tank = Taking damage with Glass Armor, Heavyglass Armor, Shield Spell/Scroll/Tomes, Ring of Shielding, Frost Charm, Potion
You can use your own bomb to tank a hit if there's no convenient enemy nearby.
You can kill miniboss while falling trapdoor to negate trapdoor fight with bomb/spell/item.
Especially useful on Eli.
You can negate damage from spiketrap with food/heal spell.
However, it requires subbeat-timing (late to spike -> early food/heal), which is difficult to do consistently.
You can use bouncetrap to farm lep golds.
Especially useful on:
Monk/Coda score: You can use throw (or fireball for horizonal bouncetrap) to get lucky charm afterwards.
Duping score: You can bouncetrap-dupe lep gold a lot afterwards. There's a technique called "Ocre strats", which allows you to kill lep on the bouncetrap with freeze tome/scroll (useful on Bard score). See here.
↓biggiemac42's great video about (classic) duping
You can use bouncetrap duping on single tempo character with late-beat input (or with heart transplant) too.
You can use floating/heavy enemies to safely escape mousetrap (or go through single bouncetrap).
You can also use most of non-floating/heavy enemies, but sometimes you take damage in this case.
You can place 2 bombs via bomb trap (but takes damage).
Especially useful to open an urn in early-game.
On Dove, if you hit lep with bomb trap bomb without aggro, you can get gold piles from lep.
To prevent aggro, you need darkness + no torch, no torch + no wall torch nearby, or warp trap nearby.
↓hokuho's awesome video
You can use scattertrap to refill heavyglass armor.
(If you have more then 3 heavyglass stack, it will decrease to 3.)
You can break tempo up/down trap to make it permanent with a bomb, or by using certain enemies (ex. elemental, beetle, hotdog, klappa, goolem, waterball).
To make the song faster for speed, or slower on Coda.
You can step into stairs as early in the beat as possible to save small time.
Game always spawn minibosses on stairs (except additional ones from boss shrine/hardmode), and they won't move unless activated/aggroed. You can snipe them with fireball/staff/rifle if you can locate stairs with map.
Especially useful on Z5 with darkness shrine activated.
You can also use metrognome (as an additional miniboss from boss shrine/hardmode) to telefrag-kill another miniboss on stairs for the same reason.
You can use stairs heal twice if there's an ice next to stairs.
You can use stairs heal as many as you want if you have blood/war drum or leaping/lunging, on Bard or with heart transplant.
You can use dash spell on stairs to negate the damage while enabling the dash at the start of the next floor.