Zelda II: The Adventure of Link
There's a trope, a joke among video game enthusiasts that sequels to original titles in the NES era (the second game in a series) is drastically different than the first. Castlevania II: Simon's Quest takes Simon out of the castle and layers RPG elements and exploration on to the original Castlevania. Super Mario Bros. 2 swaps out the Mushroom Kingdom for an Arabian theme, adds in characters, and makes it so jumping on enemies (a key mechanic in the first game) no longer kills them. And Zelda II: The Adventure of Link goes very far afield from the first game -- so far that, where it not named "Zelda", you'd never realize the two were related.
Now, some of that is just because of the US perspective of those series. As discussed elsewhere on this site, Castlevania II actually shares a lot in common with a side-release/remake of the original game, Vampire Killer including it's emphasis on exploration. Sure the RPG elements were new, but considering the direction Konami was going, Castlevania II seems like a half-step evolution instead of this weird off-shoot.
And while Americans may think Super Mario Bros. 2 seems like a weird game, but that's only because we didn't get the same game that Japan got -- they had what was, effectively, a harder "expansion pack" for the first game that used the same graphics and engine but completely different, much more difficult levels and a few new odds and ends (this game was released in the US as the Lost Levels as part of Super Mario All-Stars on the SNES). The Mario game the US got was original called Doki Doki Panic, and Nintendo just reskinned the heroes to be Mario, Luigi, Princess, and Toad (our version of the game came out overseas as Super Mario US
Zelda II, though, is a different beast altogether. As a follow up to the original Legend of Zelda, the sequel was developed specifically to be as different from the first game as possible. While both games feature a top-down perspective for their overworlds, Zelda II ditches that perspective for all encounters, towns, and dungeons (which makes up close to 90% of the game). Those used to the play mechanics of the original were in for a great surprise with the sequel.
Many other elements are different between the two games. Zelda II introduces towns, as mentioned -- pleace to go to, get information from townsfolk, learn magic, and gain healing. The world of Zelda II is much more populated than the first game, making it feel like a bustling place (and not just a creepy post-apocalyptic world filled with monsters from the first game -- and intended consequence of the limitations of an early NES game to be sure). The game also introduced RPG like elements, allowing Link to gain experience and spend those points to bump up his attack strength, magic points, and health.
Sure, a few elements do carry over from the first game, like dungeons and treasures inside them for the hero to collect (and then use down the line to explore further afield). Heart containers also make a return, helping the player extend their life bar further (on top of the bonuses gained from experience). But make no mistake -- to an outsider Zelda II barely seems like a Zelda game.
Of course, with time many of the elements of this game would influence later games. Magic became integral to the series after this game, and while the spells differ from game to game (not ever really reusing the spells in this game), they are just as necessary for completion as they were in Zelda II. More importantly, though, is that once the series moved into 3D, fully explorable worlds, the weird side-scrolling mechanics of Zelda II felt less odd-ball and just ahead of it's time. Zelda II may not feel like the top-down games associated with the series, but maybe it just knew where the industry was headed...
Similarities to Castevania Games
While it might seem weird at first to discuss Zelda II and Castevania in the same article, there isn't as much different as you might first think. Sure, the graphics aren't really the same, and Castlevania has never resorted to an overworld in the same way that Zelda II (or any Zelda game, for that matter) does, but once you set those points aside, Zelda II begins to feel of-a-piece with many of the games in the series.
For starters, Zelda II looks and plays like a kissing-cousin to Castlevania II. Both feature fully explorable worlds including towns and dungeons (although they're called "mansions" in Castlevania II). Players have to talk to townspeople to get clues and new abilities/items required to further explore the world. Many secrets are also locked away in towns and other out of the way nooks, meaning players will spend many hours trying to find everything they need for their quests.
And while experience systems may seem weird in extensions of Zelda and Castlevania, both sequels use them to good effect, forcing players to balance their needs versus how many experience they have available (although Castlevania II is a touch more unforgiving, both with how you have to spend those experience and the fact that there's a forced time limit on the game Zelda II doesn't have).
Looking further out into the series, Zelda II shares a lot in common with the full Metroidvania entries as well. Heart containers and magic containers feature in Zelda II just like the Health Max and Magic Max items from the modern Castlevania games, and it's not hard to seem a similarity between the fairy spell (which turns Link into a little fairy that can fly around the dungeon rooms, getting him into places he couldn't first explore) and the various bat/mist/wolf transformations Castlevania heroes utilize.
Like many Metroidvania games, too, Zelda II doesn't require you to explore every nook and cranny of the dungeons to complete the game. Sure, bonuses, more experience, and extra items may lurk in far off corners, but a faster, more linear path can be taken to get in and get our quickly.
For many reasons, then, Zelda II would seem deserving of inclusion in Metroidvania cannon. It may be a bit of an odd-ball for it's own series, but it's a game any fan of Metroidvania should take the time to explore.