The game is set in a fictional future during a war between the Earth and the Moon. The player is part of the Moon's forces and must invade enemy territories to gather reconnaissance and enemy weapons. The music of the game was composed in electro/techno style by Kenichiro Fukui and was published in Japan as a soundtrack album. The game received positive reviews from critics, who praised its gameplay and graphics, but felt the game's short duration and lack of two-player mode were minor flaws.
Einhänder is a shoot 'em up game of the scrolling shooter subgenre, in which the player controls a fighter spacecraft that must destroy enemy crafts in side-scrolling levels. The game's graphics are in "2.5D"; that is in 3D with occasional shifting of camera angles, but with player's movement restricted to a two-dimensional plane. Pre-rendered full motion videos are used to illustrate pivotal scenes between stages.
The player can chose between multiple difficulty settings and three different models of "Einhänder" fighter, in addition to two secret ones. The Japanese version also features a difficulty setting called "Free", which grants unlimited continues but disables the score. The player's fighter can move at different speeds and is equipped with a default machine gun as well as a manipulator arm, which is used to collect gunpods left by destroyed enemies. Gunpods have varying amounts of power, ammunition and range, and include cannons, guided missile launchers, aircraft-sized lightsabers and other types of weapons. Gunpods depleted of ammunition are discarded. They can be switched between an overhead and below-the-belly position, causing them to fire at different angles or directions. Each stage has a middle boss and an end boss (except for one), which often possess a main core and individual parts that can be destroyed.
The game's score system is based on a multiplier bar and a hit gauge counter. As the player destroys enemy cores, the hit counter increases. Points awarded for destroyed cores are multiplied by the value of the hit counter. The counter decreases when the player stops destroying cores. However, if the multiplier reaches a certain amount it will begin to flash, awarding large point bonuses for any cores destroyed during this short period of time.
As the player progresses through the game, they are relayed command orders and objectives dispatched by the Selene military satellite, Hyperion. The orders are synthesized by the on-board central computer EOS, which also records and relays the players flight and battle record data back to Hyperion. After completion of the last objective, the player is informed by Hyperion that the battle pattern of the latest EOS unmanned fighter has been completed according to the data from their battle record. The player is given the honor of being the target of the last EOS test as reward for their heroic efforts and that upon the player's death, they shall advance two classes and be awarded the Sirius decoration. After surviving the intense barrage of artillery fire of the EOS test, the pilot wonders why they must be terminated by their ally and questions their military leaders rationale for the war.
The game's narrative then skips to one month later, when the pilot re-emerges in space flying an armed Einhänder spacecraft. A message from Hyperion communicates that the player is committing an act of treason and must remove their armament and surrender. Nevertheless, the player fights their way through Selene fighters and faces the Hyperion, the game's final boss. The ending sequence depicts the player's spacecraft damaged and drifting in space. The pilot engages its thrusters and dives into an army of Selene spacecrafts with the Moon in the background. After the end credits, a lone Einhänder spacecraft is shown powering up.
The American publisher Working Designs was interested in publishing the game in North America for its Spaz brand of shoot 'em ups, but could not as Sony Computer Entertainment used their priority over Square titles to publish the game themselves. For the North American version, the speed of the arm switching was increased and some power-ups were altered; the game mode called "Free" was removed; and the Gallery was given a modified interface and different pictures than those from the Japanese version. The game was not released in Europe. On June 25, 2008, Square Enix re-released the game as a downloadable content for the Japanese PlayStation Network.
The music of Einhänder was composed by Kenichiro Fukui. The game was the first Square title to have a score from the techno/electro genre. Several tracks make use of subgenres of techno, such as progressive house, or other genres like hip hop, piano-based music or opera. The soundtrack of the game was published in Japan by DigiCube, a subsidiary of Square, on December 21, 1997, and was reissued by Square Enix on July 18, 2007. The final track, titled "Beginning", was also featured on Square Enix Music Compilation 2, a compilation album published on May 1, 2008 for members of the Japanese Square Enix website.
In Japan, Einhänder had sold 50,000 copies three days after its release, and sold 100,000 copies as of February 1999. The game received positive reviews from American gaming publications, which stated that the game was especially notable for a shoot 'em up as it had no strong competitor on the PlayStation at the time of its release. The website Allgame referred to the game as the best side scrolling shooter "to come out in the post 16-bit era of video gaming", while the British site Eurogamer called it "the most successful of Square's expeditions into non-RPG territory". In 2007, IGN ranked it first in a top ten of the best 2D space shooters.
The gameplay was praised by critics. Allgame lauded the game's intense action and diversity of spacecrafts, the gun pod system and the fact that bosses are composed of different destructible parts. GamePro noted that while the gameplay scheme is common to other shoot 'em ups, the title's "thumb-busting" action and variety of weaponry made for an enjoyable experience. The site called diversity "one of the game's biggest assets". GameSpot further called the game's play mechanics "finely tuned" and featuring a "fascinating" plot, while IGN felt the tilting camera angles were one of the features that make the game "so exciting".
Concerning the graphics, GameSpot, Allgame and Official U.S. PlayStation Magazine appreciated the level of details and effects used in the different stages, as well as the large size of the bosses. IGN noted that the game's 3D graphics made it "substantially better" than most of the other, sprite-based shoot 'em up games of the time.
The soundtrack of the game was praised by reviewers such as Eurogamer, IGN, Soundtrack Central and PlayStation Magazine. GamePro noted that the techno music and sound effects fit the stages "perfectly". However, GameSpot felt that the quality of the music and sound effects were "good" but globally not on par with that of the graphics. The game's short duration and lack of a two-player mode were noted as the game's only flaws by Allgame and GamePro. GameSpot estimated that the game demands "just over an hour" to complete.