The one puzzle that no NPC gives you any hint at is how to fight Iron Knuckles. I think you're supposed to guess at the answer after having seen an entirely different trick work, one that doesn't become available until the third dungeon and is so finicky that you can easily beat the game under the impression that it doesn't work—but they're one of the most common enemies and the game can get miserable if you don't learn how to beat them. I would have said something in the manual.

Other than that Zelda II's only flaw is an overly linear overworld, which is understandable as a reaction to its overly cryptic predecessor. Its dungeon design is unmatched in the series and maybe in the genre. It takes a simple, rewarding, fast combat system and pushes you to engage some enemies while dodging others; what higher praise could I give? The magic and 1-Up systems suggest some interesting routing too.