I don't think there's a reasonable way to explain touhou any more.
>Take Galaga, right? And add a hell of a lot more bullets and stuff, and they go in all fancy patterns and all that. Oh and everything is a girl, probably with some fancy hat or a weird dress or that kind of thing. They're kind of likable and banter between fights, so it's like one of those American action movies where they yell at each other and then BLAM BLAM BLAM and the walls explode and BLAM and you can't see anything, but with cute girls. There's also some good music, to go with the games, and the characters are likable, right? But there are a lot of games, like, 15 or so, so you have a lot of characters and a lot of music. And then the fans come in, liking these characters, and they say "wow, your art sucks, [ZUN,] I'm going to make it better" and they do, and then there's a bunch of high quality art like wow what don't you have other things to be drawing but no this is cool now we can see these cute dresses with more than 8 pixels on them. The musicians, too, the music guys get in on this, they're like, wow, this music is good, but it's kind of raw, so they rework and remix and remaster and reinstrument and retune them all. Every single one. Probably. A couple of them get really popular, some music videos, because people like good music with cute girls. And the creator, this ZUN guy, loves it. He says, yeah, I created a world, you can do whatever you want with it, I'm not going to stop this, this is good. So there are secondary works EVERYWHERE and it's like a self-sustaining system. It's died off a little, I think, but tons of doujin manga and games and music and art and body pillows that a good part of the fanbase isn't even there for the games. It's actually pretty hard to only care for the games, actually. It'll just keep going. The characters, the scenario, the music, it's all good stuff.