The four fighting games of the Touhou Project are co-developed by doujin circle Twilight Frontier/Tasogare Frontier, who have also developed several fangames for Touhou Project.
The characters Reimu Hakurei and Yuyuko Saigyouji appear in Square Enix's game LORD of VERMILION Re:2 as guest characters. The flavor text for both the characters' cards was written by ZUN.
Despite being the main character of the series, Reimu Hakurei does not appear on the icon of any main-series Touhou games for Windows. The main games' icons instead depict Marisa Kirisame. This includes the icon for the spin-off Shoot The Bullet, a game where Marisa isn't playable.
In October of 2010, Guiness World Records had recognized Touhou Project as the world's most prolific "fan-made shooter series."