Everything!
I totally LOVED its music, always unique and always wow-ed me, from the musical palindrome of Koishi's theme, so such melacholic Heartfelt fancy, there are always something you would discover every single time, it's an neverendng rabbit hole.
The lore itself was pretty interesting, even when the generic plot of the games revolves around incidents, there are always interesting things to go on, and somehow, some incident could be connected to each other
*Cough *Cough Moriya Shrine *Cough *Cough Occult balls
also, the lore is very diverse for an STG games, the hint of legends that have been fused in, you could clearly see ZUN really knows it's Youkai.
The game itself was very creative, I must say, the patterns could BE resembling things, objects, or even some evens, Raiko's Danmaku could follow the rythm of the music, Sekibanki chases you with her head, Clownpiece used moon (1CC Ruiner), Sukuna makes you big, Seija flips the screen, Sakuya Stops time, and many many many many other things.
The fangames, fan-created stuffs is all diverse, ranging from the normal STG, to RPG Pokemo-ish games, Touhou has a very dedicated fanbase that has been there for like, ages, the fanmade songs are a thing too! There are plenty genre to choose, from classical music, Death metal, Funkot, heck even Gamelan (Traditional indonesian orchestra)
The character itself is pretty much has interesting plot to dig in, like a normal fairy, to the pure-loving lunarian, it's so diverse, I began to know every single youkai listed on Youkai.com and its legend, you could also learn some hints of history behind them, Like the Fujiwara clan, or the poet Saigyou, even the tale of princess Kaguya and he legend of the great Suwa war, so many to learn from a single series!
There are so many things great about this fandom, I can't put it into words.
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