Astyanax is a side-scrolling platform action game set in a fantasy world. The hero sets out to slay the evil wizard terrorizing the land. He is armed with an ax and also has a magic lightning spell that can be powered-up two times. Magic upgrades and health potions can be found in small stone pillars. The player needs to beat monsters in a number of locales, jumping across pits and fighting bosses at the end of stages. Two-palyer co-op is supported, with the second player being palette-swap of Player 1.

The game was designed by Tokuhiro Takemori who previously worked on Legendary Axe, and the basic game mechanic is similar -- in order to deal heavy blow with the axe the player needs to stop swinging it for a while. The NES version released after the Arcade game, while generally having same gameplay, features completely different level layouts, enemy designs and story.

In the Arcade version, the plot is fairly typical-a battle is being waged between demons and people, and the people are not able to stand up to the demons. One day, a brave young man named Roche receives divine revelation from God and is granted a mythical Fire Axe. Roche embarks on a journey to the Castle of Algerine to defeat the evil demon overlord Argos and restore peace and happiness to the world.

The plot of the NES version is entirely different from the Arcade version. Astyanax is a 16-year old boy who attends Greenview High School. He has been having a recurring dream in which a young woman is calling his name. One day while on his way to class, Astyanax is transported to another dimension. He meets the fairy Cutie, who explains that he has been summoned to rescue the ruler of the kingdom Remilia, Princess Rosebud-who has been captured by the evil wizard Blackhorn. Armed with the legendary Fire Axe, Astyanax sets out on a journey to Blackhorn's lair to rescue Rosebud.