Metroid II: The Last Chozo Expansion Set
How to dechiper the weird alphanumerical code after credit rolls
When you finish the game, after the credit rolls, some sort of long serial number/key will scroll up. This serial key is "HCR435-3H7-8M08-DN4-HPR0M-1148-5M00R-07-DN1F-3H7-754L-73RC357". It is leetspeak (1337) spelt backwards. Basically, swaps each words individually, not the entire sentence, then change the numbers like this:
And you will get "SEARCH-THE-BOMB-AND-MORPH-BALL-ROOMS-TO-FIND-THE-LAST-TSECRET". Since there was a typing error in the last word, it will say "TSECRET" instead of "SECRET". This is some sort of message telling you to check the areas in the game where you found the Morph Ball item and the Bombs item.
HARD Difficulty mode
To unlock "HARD" mode, finish the game in "NORMAL" difficulty setting.
Secret Chozo Hideout and Inferna rooms
In the Expansion version of this game, there are new rooms hidden above the one where you get the bombs. Simply jump and shoot the top most block you can shoot to the right, jump in that hole and bomb the area. This will open a secret path into the ceiling, leading to a new room with a locked door. There's a destroyable block at the top allowing you to go through the pipe above the door, leading to the next area (through the walls) then the one at the top. Continue your way through there. Eventually after a save point, you will reach dark rooms where you can't see anything but hatches, enemies and if you don't fall through invisible holes, an energy tank. In the second dark area, you keep going higher and higher and eventually end up outside of the Chozo hideout but to the East of that huge hole leading to Inferna. These areas hides 4 new energy tanks and a secret boss not avaliable in Metroid II: Evolution.
In the Expansion version of this game, a new power-up was added in new rooms avaliable using bombs in the room where you get the Morph Ball item (after beating Kraid). Go through all the fake blocks to eventually reach a weird-looking room containing the armor. The armor reduces every damage by half.
Chozo Hideout secret boss: Metroid Omega 2nd Fight & GODLIKE difficulty mode
Once you beaten Boss 3, Metroid Omega, a secret boss fight is unlocked in the new Eastern part of the Chozo Ruins outside. Simply go through the secret rooms mentionned above, and when arriving outside, go to the top and continue to the East. You will see some sort of message in the sky made of blocks saying "SAVE YOUR GAME NOW" and eventually reach a strange looking save room. At this point you should follow what this message said, else...
The room after the save point is where is located the boss. If you beaten Boss 3, a Chozo will stand into the room talking to you saying Metroid Omega (Boss 3) is back and you must defeat him before he revives again then defeat Mother Brain. The secret boss is the same boss as you fought in Inferna, except that it is considerably harder, unless you are in EASY mode. The boss moves 4 times faster, has more energy and can do about 100-200 damage when he hits you in certain difficulty modes. If you beat this boss, it unlocks "GODLIKE" difficulty mode the next time you save your game, which is 4x harder than "NORMAL" difficulty and 2x harder than "HARD" mode.
Go through enemies
If you are right next to an enemy and move in the opposite direction, you'll pass through the enemy without taking any damage. The same will happen with bullets that you shoot at enemies (they won't take any damage). This glitch works in both versions of this game.
Skip boss 2 and go straight to Boss 3 with no Varia Suit
To get to boss 3, Metroid Omega, you must take the left path after entering Inferna from the Chozo Hideout elevator. However the entire area is filled with lava. In the original version of Metroid II for calculators (Evolution), even with all 10 energy tanks, it was impossible to go through the entire area without the Varia Suit, as you ran out of energy before reaching the end. However, in the Expansion, there are 14 energy tanks as well as the new secret armor that halves damage Samus receives. In addition to that, there are now difficulty modes including "easy", which halves damage received as well. This was not thought about during the game update and because of this, you can now go through the entire area without the Varia Suit.
Right when you will arrive in the room where you meet Boss #3, the Varia Suit item will instantly appear in your item list in the HUD. Basically, once you got the bombs in the upper area of Inferna and got either all energy tanks or armor (or both in Normal difficulty mode), you can skip Boss #2 and the Varia Suit room and just go straight to Boss #3.
Lava damage glitch
If you go into lava (not acid) without a suit, if the room has no enemy when you get in nor acid, you don't take any damage. This occurs in only the following places in the game: The last two lava rooms before reaching the elevator that leads to Bottom Inferna (see glitch above), as well as all the areas before the final boss that has lava.
Acid (trip) glitch
This glitch only works in EASY difficulty mode and is much harder to trigger than the previous one. It is another glitch resulting from the addition of EASY difficulty mode and the new power-ups. After you beat the final boss Mother Brain, Acid will start rising from the floor, faster and faster, and you must get back to your Spaceship before you die. Eventually the acid fills the entire screen. However, after beating the final boss, you can still go anywhere on the map, unlike in most Metroid games, except that there are no more enemies and you cannot save. In easy mode, if you got the Armor and all 14 energy tanks (and are very good at the game), you survive so long that you can even reach that Inferna lava-rising room that looks slightly like the Lower-Norfair one in Super Metroid for the SNES. Entering that room will reset the acid level to what it is normally in that room when entering it, but it will rise twice faster. If you manage to get through the entire area, lava will completly stop rising and behave normally. However you will still not be able to use any save point and using the spaceship one will end the game as it was supposed to after beating the final boss.
Metroid Omega resistant against Wave Beam?
During the secret boss fight mentionned earlier, regardless of if you got the Wave Beam or not (found after Boss 3), you will still do the same amount of damage to Metroid Omega, even thought the regular and long beams are supposed to be twice weaker. This is because this game totally disregard which weapon you have during boss fights to calculate damage, as in the original version, such damage calculation was not needed during bosses fights and this wasn't addressed when the new boss was added in the Expansion Set.
Game world map:
Here is the map from the original version of the game, Metroid II: Evolution. It's similar to the above, but excludes some areas that got added in the expansion set: https://img.ourl.ca/metroid2map.gif