In 1998, Team Shanghai Alice, a one-man developer, released Touhou Project also known as Project Shrine Maiden, a fan-made shooter series, for the Japanese PC-98, a 16-bit microcomputer.  The first game of the series, Highly Responsive to Prayers, released in 1996 was similar to Arkanoid, an arcade game developed by Taito in 1986 which expanded upon Atari’s Breakout games.

The first characteristic that differentiates Touhou from shooter games like Arkanoid is its “bullet hell”, or “manic shooters” element. This means that the player does not only need to be aware of targets that come in their direction, but they require the ability to dodge a high number of projectiles coming from all directions. This adds a layer of difficulty to the game and makes it more challenging from other shooters.

Gameplay Video

Touhou is also unique for its character depth. The player controls a human character instead of a spaceship (Space Invaders) or a ball (Arkanoid). The main protagonists of the series, Reimu Hakurei and Marisa Kirisame are given their own personality and traits, which I believe might create a connection between the player and character. For instance, Reimu is known to be optimistic and a bit curious, where Marisa is described as being straightforward and has a mania for collecting things (might be a hoarder).  

Another aspect that makes Tohou different from other shooter games, is that it was the first bullet hell in introducing a storyline. The plot is introduced to the player through conversations between characters during action interludes throughout the game. Usually at the end of every level, bosses and the player character would engage in conversation, presenting a conflict between them and the reason for which the player needs to defeat them. I believe that this adds a different goal to the game, where the player does not only go through levels in order to obtain as many points as possible, but to also get deeper into the game’s story.

Story Presentation in game - Remilia Scarlet’s Story Video

Up until now there have been multiple releases of the game, the last being Impossible Spell Card (May 2014). In each of these players can choose to play as Reimu, Marisa or some of the bosses that had been presented in previous games.

I believe that Touhou changed the concept of bullet hell games by introducing a set of original characters and compelling stories told in every game. It proved that shooting games could not only be about taking down aliens, but also they are capable of presenting a story of their own along with the gameplay. The game also possesses one of the biggest fandoms in Japan, which shows how much influence the game has had throughout the years.

Sources: 

http://en.wikipedia.org/wiki/Touhou_Project#PC-98_predecessor_games

http://en.wikipedia.org/wiki/Shoot_%27em_up#cite_note-lecture-7

http://en.wikipedia.org/wiki/Arkanoid