Crystalis was SNK‘s response to Nintendo‘s Zelda games back in 1990, it being an action adventure with real-time combat, just like Zelda. But Crystalis is definitely something more than simply a Zelda clone – it is one of the best games on the system and a great game in its own right.
That said: Crystalis didn’t set the world on fire when it was first released, but is now considered a retro-gaming classic.
Definitely worth a play if you’ve never heard of it. There’s also a Game Boy Color version, which was released ten years after the NES original, and is somewhat different and regarded as inferior.
More: Crystalis on Wikipedia