I first got Crystalis when I was 4 years old, and couldn't make it past the windmill; sold my copy. At 12, I saw it at a flea market and thought "I could probably beat it now" but barely got any farther. Here we are, almost 20 years later, and it was chosen for the RetroActive, this had to be the time for me to beat it.
It's a visually impressive game for the NES, and I liked the music pretty well. The world building is nice, and the map is robust. The attack mechanics offer a flexibility that makes combat feel more satisfying than Zelda 1. It turns out killing rooms aren't so bad when the combat is more fun. The puzzles, on the other hand, are much more cryptic. It's almost as if they assumed players would buy a guide or call a help line. The gating is inelegant in a number of ways, from a sleeping character only spawning after you buy the flute to wake him up, to the game directly telling you that you aren't ready to climb a mountain, to the level requirement for bosses.
I'm not sure I would have powered through Crystalis if not for my life long ambition to beat it, but I *mostly* find it less frustrating than Zelda 2 (a couple of bosses not withstanding) and more unique in it's lore. If indie devs are looking for a different retro game to reimagine, I wouldn't mind seeing some Crystalis clones. Either make the puzzles and exploration tighter, or focus more on the combat and it could be a good formula.