The Touhou Project is a series of 2D vertically-scrolling danmaku shooting games developed by Team Shanghai Alice, whose only member is a Japanese game maker who goes by the pseudonym of ZUN. They are similar to other shooting games, however, they focus more on working your way through complex bullet patterns containing anywhere from 10 to 100 bullets. These many complex bullet patterns are what make the series popular, as well as the large focus on characters and well-developed storylines it contains.
PC-98 predecessor games
The Touhou series was first developed for the PC-98 series of computers, under doujin development circle Amusement Makers, although ZUN was still the primary developer of the series. During this period, the series would establish the vertical danmaku formula that Touhou Project would be known for going forward, as well as gaining a small following through Comiket releases.
Five official Touhou games were released for the PC-98:
Following the fifth Touhou Project game, Amusement Makers began shifting development towards Windows platforms. After a few years assisting development on the Seihou Project series, ZUN left Amusement Makers and formed Team Shangai Alice. Despite leaving Amusement Makers, ZUN continued developing Touhou Project for Windows, expanding the series' audience both in Japan and internationally.
On November 17, 2017, ZUN and Mediascape Co. Ltd. released Touhou 16 - Hidden Star in Four Seasons on Steam, becoming the first official Touhou Project game to release on the platform. This was followed by a number of prior Touhou games releasing on the platform, as well as a statement by ZUN that Touhou fangames were fine to release on the platform, leading to a large influx of Touhou fangames to the platform.
Since Touhou 04 - Lotus Land Story, main entries into the Touhou Project franchise have stuck to the vertical danmaku mechanics associated with the series, although each game features minor variations on the series' formula, as well as typically featuring new mechanics and characters.
Alongside the main series, ZUN has developed a number of spin-off games in the Touhou Project series. These spin-offs range from variations on standard Touhou gameplay to completely different genres, although most spin-offs are either fighting games or photography games.
Developed in cooperation with Twilight Frontier, are the oldest and most common type of Touhou Project spin-off. Unlike most 2D fighters, the official Touhou fighting games tend to focus heavily on projectiles, with spell cards from the shooting games returning as special moves.
Developed by ZUN, the photography games play similarly to standard shooting games, with the caveats that only boss fights are present and the player can not attack. Instead, the player is tasked with dodging bosses' spell cards while attempting to take pictures of the boss. The player's photos are then judged by a number of factors, including the boss' visibility, the number of bullets included in the shot, and more.
Given the series' notable popularity, a large number of fangames have been made featuring characters, locations, music, and mechanics from the Touhou Project series. This is further bolstered by ZUN's frequent efforts to encourage Touhou fangames, including putting very few restrictions around the use of Touhou characters as well as working with Mediascape on the "Play, Doujin!" project, a publishing project dedicated to licensing and releasing Touhou fangames commercially.