A subtype of the Action Adventure genre, usually with Platform Game elements.
Your typical Metroidvania game is typically portrayed as a single large area or a set of large areas, broken up into many different rooms, corridors, and open spaces, with Respawning Enemies in most areas. As the player progresses through these areas and finds Video Game Tools (actions, abilities, inventory items), the ability of the player to navigate more obstacles in the play environment increases, allowing the player to explore a greater amount of the game, and in doing so progress the game. This makes Backtracking occasionally necessary, often made easier by opening Doors To Before. There are usually many secrets hidden around the game, some far more difficult to obtain than any item required to proceed.
It often contains mild RPG Elements as well, like stat-boosting equipment or a level system; some of these games will have multiple playable characters with different abilities and require the player to switch between them. But if not, expect to find hidden Heart Containers in every cranny and nook.
Despite the openness of the game, progression is usually linear, with the more difficult areas separated by natural barriers such as high shelves, sealed or locked doors, or other obstacles that can only be bypassed by finding specific items or weapons. Among gamers, Sequence Breaking is a common stunt used to access these areas before the player is "supposed" to. (Some games deliberately design sequence-breaking paths as well.)
The definition of this subgenre varies somewhat depending on who you ask. People seem to variably demand some or all of the following traits:
Some people say it has to be a 2-D environment; some even go as far as saying it has to be platforming.
Non-linearity of (official) game sequence, often resulting in backtracking, especially for new players.
This sub-genre gets its name from the Metroid and Castlevania series. Metroid, published in 1986, was the Trope Codifier, (though the style had previously been utilized in the Atari 2600 game Pitfall II: Lost Caverns), and subsequent Metroid games have consistently used it in all of its installments (except Prime Pinball and possibly Prime Hunters), while Castlevania largely switched to it after the success of Castlevania: Symphony of the Night. The term itself was coined by Jeremy Parish of Gamespite, who originally used the term to refer specifically to those Metroid-inspired Castlevania games. More information can be found here.
A somewhat lesser version of this was fairly popular towards the end of the NES' life cycle. The game would be separated into stages, but each stage was a wide-open, explorable area instead of a linear progression. Many of these games allowed you to revisit a stage after you already beat it.
Games in this genre tend to be a four (or three) on the Sliding Scale Of Linearity Vs Openness. Competing terms include "Castletroid", "Castleroid", "Metrovania" and "non-linear action adventure platformer", with or without capitalization.
Unfortunately, this is not a crossover betweenCastlevania and Metroid... but it soshould be◊.
Examples
All of the Metroid games and most of the recent (post-2001) Castlevania games, of course. Some people don't consider the 3-DMetroid titles to count, but even those games play out as if they were Metroidvania games with a first-person perspective. The main difference here is the environments are arranged in 3 dimensions rather than 2. Symphony of the Night isn't the first time that the Castlevania series experimented with the genre; Vampire Killer and Castlevania II: Simon's Quest shared many of the same gameplay elements, though the latter didn't have the closed complex setting typical of the genre (and the former had no RPG Elements.)
Dark Souls is a rare example of 3D Metroidvania game.
Pitfall II: Lost Caverns was probably the Ur Example. Super Pitfall as well, of course, and Pitfall: The Lost Expedition/The Big Adventure as well.
Another likely candidate is Montezuma's Revenge, also released in 1984 on the Atari 2600; though the game world is a bit small, the gameplay is familiar.
Which are, arguably, more connected through the Hub Level rather than interconnected.
The first Mega Man ZX game features waypoints that allow you to teleport to any waypoint already visited, but all waypoints must be found by exploration first (except for the isolated area that becomes the Hub Level, which gets added to the teleport list once you complete a certain early mission), and very few areas have their entrance right next to a waypoint. The first Mega Man Zero game features the same, but has a habit of automatically placing you at the beginning of the relevant area at the start of each mission you accept; which, combined with the fact that many areas host two missions, means that if you never wandered outside the Hub Level in between missions, you would never notice that most of the areas are physically connected to eachother (specifically, you would only notice that one pair of areas, plus the linearly-connected areas of the final three missions, are connected). Mega Man ZX Advent, on the other hand, is definitely a downgraded version of this genre, with at least as many missions taking place in self-contained areas as otherwise.
In addition, Zero lacks the ability-gaining that is central to the genre (some Cyber-Elves can give you permanent upgrades, but they're never needed to access areas you couldn't reach before).
Nicklas "Nifflas" Nygren's games Within A Deep Forest, Knytt, and Knytt Stories. His games are unique in that they have minimal power-ups and focus on exploration.
Kirby and the Amazing Mirror is unique in that unlike other Metroidvanias, instead of finding abilities to progress to other abilities, you have to eat enemies for their abilities which are near the parts you have to use those abilities in, and you can discard the abilities soon after. You could also get help from the 3 other Kirbies, and do things in any order, even reverse. The Great Cave Offensive in Kirby Super Star had similar gameplay, but was more linear. (Note that other games in the series, including the other components of Super Star, are Platform Games).
Some games in the Zelda series, as they share the gameplay element of granting powerups to the player character that allow him to access more places. (Sequence-breakers and speed-runners have similarly had many field days with Zelda games.)
Zelda II: The Adventure of Link may fit the 2D side-scrolling version of this trope, for those who insist that this genre must include 2D side-scrolling.
Depending on the definition, possibly all games in the Zelda series, since the leading characteristic of a Metroidvania is not its sidecrolling nature, but the hybridization of Puzzle/RPG/Adventure elements, and the use of Video Game Toolsspecifically to make it to new areas. Lufia 2 Rise Of The Sinistrals, despite having many of the same elements, cannot be considered this, since these tools are restricted to dungeons, rather than being used to move around in the world itself.
Cave Story borders on this. It hits most of the requirements of the game type except for two: it's fairly linear barring sidequests, and areas aren't as interconnected as they could be due to just using the Hub Level. It does show influence from both Castlevania and Metroid.
Indie freeware game Iji borders on this - once you clear a level, you can't backtrack, but each level is huge and there are several secret areas that require Metroidvania logic to reach - to get one Supercharge requires getting a jump upgrade, using an enemy's rocket attack to reach an elevator back to a now accessible ledge that leads to a weapon necessary to destroy a wall blocking off the powerup.
Hero Core by the same creator can basically be described as the combination of a Metroidvania and a Shoot 'Em Up.
Samurai Jack: The Amulet of Time for Game Boy Advance was a transparent wholesale ripoff of both the GBA Castlevania and GBA Metroid games. Not that it was bad...
The Nightmare Before Christmas: The Pumpkin King for Game Boy Advance was a transparent wholesale ripoff of the GBA Castlevania games. It was considerably better than Samurai Jack.
Hebereke for the Famicon (and the Euro Release Ufouria). Something about a drunk duck (hebereke translates into stumbling drunk) falling into an alternate dimension with his animal-ish friends who have to find a way back home (or so it appears). Plays like Metroid meets Mario. Unfortunately, all the sequels (on the SNES) completely abandoned this genre and are party games.
There's even a Sonic game with a Metroidvania theme—the Game Gear spin-off Tails' Adventure. By all accounts a pretty good game.
Sorta; it was divided into levels for cartridge space's sake. But everything else played out like a Metroidvania, and you could revisit levels to look for new stuff.
Sonic Adventure had shades of this (all the levels were connected through a hubworld, you could backtrack and gain various items) Sonic Adventure 2 dropped most of this.
Sonic Advance 3 seemed to be another stab at this: All worlds are connected through a hub, and different character combinations beyond the initial Tails and Sonic are needed to explore the levels fully and achieve 100% Completion.
Faxanadu - the other Dragon Slayer games also have elements of this.
Legacy Of The Wizard in particular is a much more Metroidvania-like entry in the series.
In 60 Seconds is a freeware mini-Metroidvania. As the title suggests, you get just one minute to gather all the abilities required to reach the boss and defeat it.
Knightmare II: The Maze of Gallious, the game that inspired La-Mulana.
Shadow Complex on Xbox Live Arcade has been described by pretty much every single reviewer as an (awesome) callback to Metroid and Castlevania. This was intentional: the developers have openly admitted to basing it on said games, and spent the entire first month of developmentplaying them. Even the minimap in the top right corner looks eerily familiar. On top of that, the debut article about the game in Play Magazine mentions Super Metroid 17 times. On the first page.
Legacy of Kain: Soul Reaver, although its sequels were much more linear in nature.
Mega Man Legends is a partial case, in that it's a third-personal shooter but has a very similar nonlinear explorative feel to it. With the proper special items, nearly all the underground areas can be made to interconnect too.
Exhumed, also known as Power Slave, is possibly one of the earliest examples of a Metroidvania FPS, predating Metroid Prime by almost a decade.
Banjo-Tooie borders closely on this, and perhaps can be used to illustrate the genre differences between this and a straight Platform Game. The levels, as in the previous game, are non-linear, large and focused on exploration and item collection, have a lot of puzzles and pathways between each other, and there are a lot of unlockable moves and attacks required to progress through the Hub Level and to complete everything. On the other hand, progress in the game is limited by having later levels mostly inaccessible until opened one at a time by turning in Plot Coupons at a central location (unless using a certain in-game cheat), so Sequence Breaking is fairly limited.
Rygar, the NES version, which has a whole series of items to collect in order to improve your climbing skills more and more, and then makes you try to remember which previous stage had that unreachable ledge.
Metal Walker, while an Action RPG, has elements of this. Returning to previous areas with more Core Units can get you items, gold, and in some cases, new Recipes and special Cores.
The NES version of Strider often requires returning to levels several times after obtaining keys or ability upgrades.
Surprisingly, the NES videogame of Rambo, which featured one of the most confusing, maze-like game worlds ever.
Teenage Mutant Ninja Turtles: Radical Rescue for the Game Boy. You start off as Michaelangelo and must rescue the rest of the gang, whose abilities open new areas to explore.
The first NES TMNT, although level-based, has explorable levels with an overhead overworld and side-scrolling indoor areas.
The Phantom 2040 videogame for the SNES and Genesis. (Different areas are connected through a World Map rather than being continuous, but it's still a good, classic example of this genre.)
◊.
