And while overall most NES games didn't use a battery, many of the most popular ones (many of the ones included in this thing, in fact) do need a working battery. Final Fantasy, both zeldas, startropics, tecmo bowl, crystallis, kirby's adventure...
Crystalis absolutely has a save/continue system. You can see some guy demonstrating the save system in crystalis in this video, complete with save functionality: https://m.youtube.com/watch?v=73za3LHWzwk
Incredible. I owned the game for years, played it a lot, beat it once, and never noticed a way to save, so I assumed you couldn't. I even had the manual!