Metroid II was released back in 1991 for the original Game Boy. The game was the first one after the original NES Metroid game and introduces many new elements, game mechanics and a graphic style that have defined the series since then. The game however does also divert from other parts in the series. Most notably is this in the story. Samus is given the task to destroy all Metroids on the planet SR388. Ridley, Mother Brain or other classic Metroid enemies don't have part in this game. The game also follows a much more linear path, you start at the entrance to a cave and then basically work your way through an underground maze exterminating Metroids in the process.
After selecting one of the three available save slots, the game starts you next to your landed space ship, similar to Super Metroid and the games that followed. Samus starts out with the Morph Ball and a supply of 30 rockets, while other abilites and upgrades need to be collected in normal Metroid style. A counter at the lower right of your screen tells you that there are 39 Metroids that you need to kill, pressing Start will show you how many Metroids are in the current level. Unlike other Metroid games, in this one there are no boss enenmies, aside from the final Queen Metroid, instead each of the 39 Metroids acts as sort of a mini boss.
The level design in Metroid II diverts from other games in the series. Instead of one huge world with frequent backtracking, the world is separated into levels that are linked together in a linear fashion. Each level contains a fixed number of Metroids that need to be killed and progress into the next area is blocked by a wall that won't disappear till an earth quake, triggered by the last Metroid killed, will remove it and allow progress to the next level. The levels themselves however don't follow a linear structure and are basically smaller side caves that allow non-linear exploration. Backtracking through all the levels back to the top is possible, but is never needed or encouraged in the game. Backtracking however is frequent on a per level basis and once you unlocked the next level you have to often leave the cave back the way you came in. The levels themselves are also filled with dead ends, so you often have situations where a Metroid is in a side cave, which you then enter to kill the Metroid and then leave it the way you game. The game does wrap itself back to your ship once the final Queen Metroid is defeated in traditional Metroid fashion. Another noticable divergence from classic Metroid formular is that the game does not have doors in the normal levels, you switch rooms by simply walking to the edge of the screen. It is only the Chozos chamber that contian weapon upgrades that are locked by missile doors.
Unlike later games in the series, Metroid II is still a map-less game, relying on the players memory, a handdrawn map or a walkthrough. The game also provides little to no hint as to where an earthquake has opened up a new area. It does show the count of Metroids still alive in this area and the earthquake is a strong graphical clue, but there is no directional hint given.
In terms of game mechanics, the game feels a lot closer to the later Super Metroid then it does to the NES original. The game introduces the ability to shoot downwards and the ability to duck. The the ability to shoot diagonal is still missing however.
The game introduces the Spider Ball, the ability to turn the Morph Ball into a sticky form that will attach to walls and allow you to roll up vertical walls or even roll around the ceiling. The Spider Ball made a reappearence in the Metroid Prime series, but so far Metroid II is the only one of the 2D Metroid games that provided this ability. The controls of the Spider Ball are a bit trouble some, as it will only work when you press into the proper direction, which when you ball is right at the corner of a wall becomes trouble some, as it is hard to tell if it is already on the vertical part of the corner or still on the horizontal one. The Spider Ball also rolls around a little slowly, which given that you sometimes have to travel quite long distances with it, can get a little annoying.
The Metroids in this game not only come in their common jellyfish form, but in different stages of their lifecycle namely Alpha, Gamma, Zeta, Omega. Each of which presents basically a harder form that is more difficult to fight. While those other lifecycles make a short guest appearence in Metroid Fusion, Metroid II so far is the only game in which the different stages of the Metroid lifecycle played a major role.
One of the weird parts of this game is how it handles enemy activation, while it is normal for console games to only activate enemies that are close to the screen, here only enemies are activated that are fully on the screen. In the Metroid fights this has the effect that you often have the Metroid almost outside, but still visible on the screen, yet he will not be activated and stand still. This basically becomes a boss tactic, you try to keep the Metroid outside the screen, then jump into its direction at the right high fireing a missile, then back out again to freeze the Metroid with the help of the screen border. This limitation to the screen area also comes into play with destructible sand tiles, that respawn instantly when out of screen, which feels a little weird, as it often happens by accident, blocking a just cleared area.
The game features savepoints similar to the later games in the series, but due to the layout of the levels you don't come across them often enough, forcing you sometimes to backtrack to an earlier savepoint to save. Savepoints in this game also don't replenish you health, so you have to rely on health and missile refill points, which however are very sparingly as well or you have to shoot enemies, which while working, can be rather time consuming.
The graphics are for the technical capabilities of the Game Boy quite nice, featuring a much more zoomed in view with very large sprites compared to the NES Metroid game. This zoomed in view however comes at a price, as it provides much less overview over the current room, making it sometimes hard to orientate oneself. The graphics are however mostly limited to cave tiles, while there are different kinds of cave tiles provided, the more industrial design of later Metroid games is mostly missing, making the game lacking in noteworthy locations. The lack of color or good animation also makes it nearly impossible to distingish lava from water in this game, but that never really becomes much of an issue. Playing the game on the Game Boy Player will give the game a custom palette, providing Samus with proper coloring and breaking the regular four color limit on the original Game Boy.
The music in Metroid differs from the other games, but is overall quite nice and different tunes for different areas keep the music from getting to repetitively
Overall Metroid II is still a very fun game, but just like the original, it suffers form the lack of a map screen. The game also suffers from a general slowness, not only is the jumping a little slow, but especially the spider ball just moves to slowly. The story, presented completly without cutscenes, on the other side is rather nice, even so it is very simple, as it moves the Metroid series away from the normal boss fights, giving the mission a clear purpose and adds some moral ambiguity in the end, when you destroy the last Metroid and then a newly hatched baby Metroid follows you peachfully, even helping you, for the rest of the way back to your ship. A classic countdown is missing in that sequence.
The game is around four hours long when played casually with a map. The classic Metroid special ending is unlocked when finishing the game in under three hours. A counter for how many items one has collected is missing, so one doesn't get anything extra when collecting 100% items.
PS: There is a fan made remake of the game in works, using what looks like mostly recycled Metroid: Zero Mission graphics.