Review and Overview by Mike Finkelstein
It's hard to talk about Metroidvania games without first discussing the game that started it all. It's not often that you have a game whose title effectively becomes the name for a whole genre of other games, but that's the legacy of Metroid. The game is, essentially, a run-and-gun shooter but to describe it as such belies what the game is really about. It's a moody, sci-fi, explortation-heavy adventure where you never know what might be beyond the next room... or if you've even been in that room before.
Coming at it with fresh eyes, trying to look at it as if you were someone that has no clue what the game is about, Metroid is a slow burn of a title. Your space-suited hero (Samus Aran, who we eventually find out is a woman, but even that detail is hidden from us) is dropped down to the planet Zebes and has to defeat the evil within. That's your whole mission and figuring out what to do, where to go, and how to accomplish it is entirely up to you. You have one goal, a non-linear path to take to accomplish it and, up until the last segment of the game, as much time as you want to finish out your mission.
Metroid is not a forgiving game. Players that may have explored later entries in the series (or any of the Castlevania titles in the genre) may be used to regular health increases, healing magic, and save points -- -very little of that is here. Sure there are Energy Tanks, but a sparing few of them, and the enemies later in the game hit like trucks. You can't really heal yourself, instead limited to the sparing health drops enemies will give you. And don't expect save points anywhere, as there are none in Metroid, just passwords to get you started again (although you'll always begin back at that first screen, so you better learn the layout of the planet quick).
You should probably expect to get lost pretty regularly as you explore Zebes -- there no auto-map to save you here, and no fast travel to help you find your bearings or get back to areas you recognize. Those are modern conveniences that the original Metroid doesn't have time for. It's not going to explain to you where to go. It's not going to tell you all the items you need or hint as to what you might need to find. If you want to get to the end of the game and kill the big bad, Mother Brain, you're going to have to do it all yourself.
Of course, that sense of mystery, the tension you felt every time you went into a new hallway, the game making a constant alert noise because your health was low, that was all part of the charm of the game, the reason you kept coming back. It was hard, but it was still fun. Which was, of course, the Nintendo magic.
Similarities to Castevania Games
The statement is really more accurate to say "how is Castevania similar to Metroid?" The first game in the Metroid series helped to establish the basic outlines of the genre. A lone hero exploring a giant fortress where the zones blend from one to the next creating a world you could explore seamlessly. The collection of items (including the ever popular Ice Beam/Freeze Ray) to aid in your exploration. The goals you have to clear to unlock the final area(s). All of that is built into the first game.
That said, much of what we really expect from a Metroidvania game comes from the third game in the series, Super Metroid. Everything good about the first game was perfected in the third titles in the series, while Super Metroid added more refinements, more improvement, and more quality of life bonuses that cemented. But all of that grew out of the first Metroid and, to be honest, when people complain about how Metroidvania games are too easy, they focus too much on story or directed goals, that's because those changes move the genre further away for the pure simplicity of the first game in the genre.