Altered Beast is a side scrolling, platform, beat 'em up game that puts the player in control of a centurion who had died in battle. The centurion has been raised from the dead to rescue Zeus' daughter Athena from the demon Neff. The player battles undead and demonic hordes, controlling the shapeshifting hero. He must fight through several levels in order to save the kidnapped goddess. Although 'Centurion' was a rank in the Roman Army, the game takes place in a setting resembling Ancient Greece, complete with gods, temples and ruined Ionic columns.
The player must battle armies of fictional and undead creatures to defeat the evil Demon God Neff, who is holding Athena captive. Along the way, the player has to obtain "Spirit Balls" (power-up orbs which increase his strength and size) from defeating white two-headed wolves (It was blue oxen in the DOS version. It is hinted that this wolf might be the Orthrus dog of Greek Mythology). These enable the player to turn into a superhuman (stage one: "giant man", stage two: "superman"...). When three are collected, the hero transforms into a beast with exceptional abilities.
The game contains several levels which the player must battle through, however the player is not required to reach the beast form in order to complete the level. At the end of each level is a "boss" creature, which is Neff himself in different forms. Before Neff transforms at the end of each level, he says "Welcome to your doom!". The enemies the player encounters differ depending on the level as does the beast the hero transforms into. These beasts include a werewolf, a thunder dragon, a tiger man, a bear, and the more powerful golden werewolf (other beasts can be seen in the Japanese Famicom version and the Game Boy Advance version). Each beast has its own special abilities, such as the dragon's ability to fly. Between each level are small animations giving the player glimpses of Athena's peril.
