Sonic the Hedgehog 2 (Genesis)
|Sonic the Hedgehog 2|
Developers: Sonic Team,
Sega Technical Institute
This game has unused areas.
This game has a prototype article
This game has a bugs page
This game has a prerelease article
Most of the removed zones can be found here in some form, except for Blue Lake (which was present but unused in the Simon Wai build). Check if there are any leftovers.
Sonic the Hedgehog 2 is the sequel to the first game, which introduces Tails, the Spin Dash, and the idea of robot Sonics. A large number of interesting things were cut from the final version, such as the ever-famous Hidden Palace Zone.
- 1 Debug Functions
- 2 Cheats
- 3 Unused Graphics
- 4 Unused Objects
- 5 Unused Score Tally
- 6 Unused Audio
- 7 Inaccessible Rings
- 8 Misplaced Objects
- 9 Scrapped Zone Remains
- 10 Unused Oil Ocean Zone Background Section
- 11 Oddities
- 12 Unused Level Chunks
- 13 Unused 2x2 Tiles
- 14 Early Wing Fortress Zone
- 15 Revisional Differences
Like the first game, Sonic 2 has a well-known level select:
- Go into the Options menu.
- In the sound test, play sounds 19, 65, 09, 17 (September 17, 1965 being Yuji Naka's birthday). This enables the Level Select.
- Press Start to return to the title screen. When the menu appears, hold A and press Start to bring up the Level Select screen.
Enabling the game's debug mode requires the level select to be enabled.
- In the level select's sound test, play sounds 01, 09, 09, 02, 01, 01, 02, 04 (November 24, 1992 being the date of the game's US release).
- Choose any level, then hold A and press Start. Keep A held down until after the title card pops up.
Alternatively, it can be accessed with Pro Action Replay code FFFFFA:0001.
From here, you have access to a variety of things:
- Unlimited time
- Debug coordinates (in place of score)
- Sprite counter (in place of time)
- Placement mode: Press B to enter placement mode. From there:
- Press A to move forward one object.
- Hold A and press C to move backward one object.
- Press C to place the object.
- While the game is paused, some debugging options are available:
- Press A to go back to the title screen.
- Press and hold B to slow down the game by 50%.
- Press C to advance the game by one frame.
In addition, if Sonic dies, he can be revived by entering placement mode, moving to a safe spot, and exiting. Trying to place an object in this death state will freeze the game, however. You must also be playing as Sonic, as Tails cannot enter object placement mode.
"Night Vision" Mode
Go to the Level Select screen, enable Debug Mode, then hold C and press Start when selecting a level. This will cause the console's VDP to activate a debug function: background graphics drawn behind the sprite layer will be covered with a translucent black overlay.
Moving Background Debug
Use PAR code 00EBEC:4E71 (Rev. 00), 00EBC0:4E71 (Rev. 01), or 00EBC2:4E71 (Rev. 02) to enable a special debugging feature: Hold 2P Up or 2P Down to move the rising floor / lava in Hill Top and the moving clouds in the background of Wing Fortress.
- All 7 Emeralds: To get all 7 Chaos Emeralds, unlock the Level Select screen and play sounds 04, 01, 02, 06.
- Extra Continues: For 14 extra continues, play sounds 01, 01, 02, 04 on the Options screen. This cheat makes Oil Ocean Zone music play all the time due to an incorrect sound command.
|Unused CPZ Animation||There's an unused animation for Robotnik accidentally dropping the blue goop on his own head in the Chemical Plant Zone fight. This animation has been around since the Nick Arcade prototype, but it was never implemented. There's a fix to implement the lost animation here.|
|Robotnik laugh||In Death Egg Zone, Robotnik is programmed to laugh when Sonic takes damage during the Mecha Sonic battle. However, as there are no Rings in this Zone, Sonic instead dies immediately when hit and all animation is frozen, so this is never normally seen without debug mode. In the 2013 mobile remake, this animation is not frozen when the player dies. You can also view the animation in the Sonic Jam port by playing in Easy mode, as well as the 3DS port by playing in Ring Keeper mode.|
|HPZ Icon||A Level Select icon for Hidden Palace Zone. It is not normally used in any version of the game, but Pro Action Replay code 0096D8:0303 will cause it to replace the Emerald Hill icon in the Stage Select. It also appears in Sonic the Hedgehog 3's level select, which contains icons for the Sonic 2 Zones.|
|CPZ Decorative object||Unused decorative objects for Chemical Plant Zone, stored at 0x0383-0x0393 in VRAM. These can be placed in the iOS/Android 2013 version's debug mode.|
|OOZ spring holder||A holder for the OOZ ball launcher (see below). This appeared in the Simon Wai prototype, and is still loaded into VRAM in the final game.|
|Unused Aquis graphics||An unused animation for Aquis spitting up oil, along with an oil spray.|
|Unused Balkiry frame||An unused frame of the Balkiry badnik from Sky Chase Zone with its landing gear deployed. It's possible that they were originally intended to take off from the Wing Fortress as it passed by. If a Balkiry is selected in debug mode, this sprite will be seen instead of its flying one.|
|Walking turtle||A walking animation for the turtles in Sky Chase Zone. The only way to see this sprite is by using debug mode to create and destroy a badnik while the Wing Fortress passes by. This would later on make an appearance in Sonic Mania, shortly before the Metal Sonic boss battle at Stardust Speedway Zone.|
|Picky||Picky, a returning animal from Sonic 1, is present in Death Egg Zone. However, there are no badniks or animal capsules, so he's never seen.|
CPZ Hovering Platform
This object has been around since the Nick Arcade prototype. Every 1024 frames, it will slowly rise up for 128 frames, float in the air, then come back down 128 frames later. The amount of time it floats in the air is 128*(lower four bits of subtype) frames. The upper four bits can be used to offset when it starts to rise in the air.
This crystal was used in the earliest two prototype to block off a tube in Hidden Palace Zone. The graphics are long gone, but the code is still there, such as it is. It's a simple solid indestructible object and its coding hasn't changed since the Nick Arcade prototype.
Another Hidden Palace Zone leftover. Again, the graphics no longer exist in the ROM. Its height is equal to 16*(lower five bits of subtype)+16 pixels. Placing one with a subtype greater than 17 will spawn garbage objects on the right.
Green Hill Zone Spinning Spikes
Object ID: 17
A leftover from Sonic 1. Unfortunately, the graphics have been deleted, but the coding is intact.
OOZ Ball Launcher
A ball and spring object, which can be placed in Oil Ocean Zone via debug mode. If a switch is placed near it and pressed, the ball will pop out and start rolling in whichever direction Sonic was facing when the object was placed. It seems this object would have been used to cross something, as it can be stood upon, much like the ball object that appeared in early versions of Sonic the Hedgehog. It does not collide with breakable floors or elevators, and it will go up slopes and even vertical walls without slowing or stopping. It can be seen in the Simon Wai prototype.
MZ Small Platform
Object ID: 6C Although very similar to the small platforms used in Metropolis Zone, this platform behaves strangely, usually rising at an angle.
MCZ Ring Platform
An odd platform made of rings, which can be placed in Mystic Cave Zone via debug mode. It can be stood on, and moves in a similar motion to the spinning spiked balls in the level.
WFZ Turbine Fire
If the sub-ID of the rotating platforms in Wing Fortress Zone has bit 02 of their subtype set but not 04, some kind of laser will appear (with no sound effects) when the platform spins. This object can be placed with debug mode, where it oddly appears before the used platform in the object list.
WFZ Pole Badnik
A strange object that seems to be a spinning pole, which can be placed in Wing Fortress Zone via debug mode. It's like a badnik: you can destroy it, and it can damage you, but it doesn't move.
CPZ Tipping Platform
The amount of time that the platforms in CPZ can stay flipped is variable -- equal to 10+(upper four bits of the subtype) frames -- but in the final game, the upper four bits are always set to 70.
CPZ Bubble Snakes
The blue bubble snakes in Chemical Plant Zone have many subtypes that define their movement pattern (00-0F for jumping back and forth, 10-1F for jumping between the floor and ceiling) and their length, which can be any number of segments between 1 and 16. However, only those that are 6 segments long are used (subtypes 05 and 15), leaving all other subtypes unused.
OOZ Pressure Stopper
The used pressure stopper in Oil Ocean Zone launches the player upwards. If the object subtype is set to zero, as it is in edit mode, it will launch the player to the right. This would have been used in the Simon Wai layout of the stage, but has no purpose in the final layout.
Unused Score Tally
This configuration for the score tally, replacing "ACT #" with "ZONE", is meant to be used in levels with only one act, such as Sky Chase Zone, Wing Fortress Zone, and Death Egg Zone. However, none of those levels have a score tally, so this text never appears unless a capsule is placed via debug mode and destroyed. This was later used in Hidden Palace Zone in the 2013 remake.
The song at entry #10 in the Sound Test is unused in normal gameplay; however, it will play if the corrupted remains of Hidden Palace are accessed in the final build through the use of cheating devices or otherwise hacking the game. It does not exist in the Simon Wai prototype, where instead the functional version of Hidden Palace uses the music that was later used for the 2-player version of Mystic Cave.
Disc 2, Track 37 of the Sonic the Hedgehog 1&2 Soundtrack contains Masato Nakamura's original demo version of this song, but it doesn't loop and then fade out like the Zone demo tracks; instead, towards the end of the track, the tune changes significantly and has a proper ending. This may suggest that the music was originally intended for some sort of cutscene, perhaps taking place in Hidden Palace. Interestingly, this track is referred to as "STH2 Unused Song ~ Masa's Demo version ~", without specifying its intended use, though all track titles can be taken to be indicative only of how each song was used in the final release version.
Although popular speculation is that this song was at one point intended to be used as the background theme for Hidden Palace, the 2013 remaster of Sonic the Hedgehog 2 instead uses the same music as the Simon Wai, August 21st, and September 14th prototypes, as the developers felt that it better suited the revised level. However, this track later found use in the aforementioned remaster's "Proto Palace Zone".
A few unused leftovers from Sonic the Hedgehog and other unused sounds can be heard in the Sound Test.
|Sonic the Hedgehog Leftovers|
|$29||The sound used when hitting the R, UP, and DOWN blocks in the Special Stage.|
|$31||Electricity sound from Scrap Brain Zone.|
|$38||Sounds similar to the sound of spikes popping up. This is also unused in Sonic 1.|
|$3A||The sound made while hitting the colored diamonds in the Special Stage.|
|$43||The warp noise made by a Giant Ring at the end of an act if Sonic enters it. This sound was later reused in Sonic the Hedgehog 3.|
|$47||Chain pulling sound from Marble Zone.|
|$49||Bonus point tag appearing at the end of an act.|
|Other Unused Sounds|
|$51||Unknown. Sounds like something popping or skidding on the ground.|
|$52||A low siren, similar to $53 which is used in the 2-Player mode.|
|$68||Appears to be a slightly shorter version of Mystic Cave Zone's bridges stopping in place ($69).|
Some levels have rings in places that cannot be reached, making a Perfect bonus impossible.
|Location Image||Level||Approximate coordinates||Description||Location Image (Prototype)|
|Aquatic Ruin Zone
|1FA90626||Two rings lined up horizontally. The rightmost can actually be touched without going inside the pillar, but there's no way to reach it. The pillars weren't there in CENSOR and Beta 4 prototype, allowing the rings to be collected.|
|Casino Night Zone
|027F00F8||Two clusters with seven rings each, just above the start, but way too high to reach. These rings have mysteriously been transported from Act1 to Act2, this same problem also occurs with the rows of rings that should be from a launcher.
These rings would be easily reached using fippers that are centered just below the rings. When locked-on to Sonic & Knuckles, it is possible for Knuckles to collect these rings by gliding to the far left from the highest wall.
In the August 21 prototype or final version, these flippers could be used to reach the enigmatic rings on ACT1.
|Casino Night Zone
|10700611||It may seem strange, but this row of rings should be in ACT. 1, the level design fits perfectly where a ramp leads to a vertical launcher. This failure has always occurred since the prototype of August 21st, 1992.|
|Mystic Cave Zone
|0F2B00F0||These may have been placed with the original level layout from the Simon Wai prototype in mind. In the same coordinates in that prototype is a tall pillar.|
|Mystic Cave Zone
|15880160||These may have been placed with the original level layout from the Simon Wai prototype in mind.|
|Mystic Cave Zone
|16050156||These rings may have been part of the original level layout for this Zone in the Simon Wai prototype. As there is a ledge and an empty space in the same coordinates in that prototype, a swinging platform may have been in these coordinates to accompany the rings.|
|Oil Ocean Zone
|0BC003A8||These are leftovers from the original level layout seen in the Simon Wai prototype. The same rings can be found in the same coordinates in that prototype.|
|Oil Ocean Zone
|11BF023A||These are leftovers from the original level layout seen in the Simon Wai prototype. The same rings can be found in the same coordinates in that prototype.|
|Oil Ocean Zone
|12C0023A||These are leftovers from the original level layout seen in the Simon Wai prototype. The same rings can be found in the same coordinates in that prototype.|
|Oil Ocean Zone
|Oil Ocean Zone
|Oil Ocean Zone
|02C0043F||Three horizontal slanted rings, under the floor with the ten-ring box and a cog nearby. This floor used to be a slope in the Simon Wai prototype, so these rings may have been placed with the slope in mind.|
|135000D0||These rings may have corresponded with the original level layout from the August 21st prototype. Judging by the layout in that prototype, these rings could have been reached by jumping from the nearby conveyor belt.|
|0F000169||These rings appear to correspond with the original level layout from the Simon Wai prototype. There is a ledge in the same coordinates in that prototype, so the rings may have been placed on top of it.|
|0F400354||These rings appear to correspond with the original level layout from the August 21, 1992 prototype. In the same coordinates, they would have been under a ceiling.|
Various objects throughout the levels are misplaced or in odd positions, possibly indicative of earlier level designs.
Scrapped Zone Remains
Ocean Wind Zone (?) (ID 01)
This is an empty level slot that exists in the game, which contained remnants of Labyrinth Zone in the Nick Arcade prototype. It can be accessed via Pro Action Replay code FFFE10:0100 or Game Genie code ACLA-ACD8 (AB6X-ACBR when locked-on to Sonic & Knuckles), and choosing "Sound Test" on the stage select menu (Special Stage with lock-on). This stage is an objectless version of Emerald Hill Act 1. Additionally, it has a Tails icon, much like the Wood Zone remains. Finishing Act 1 by hacking in an animal capsule will send you to Emerald Hill Zone Act 1.
Going by early concept art which shows roughly what the level order would have originally looked like, and lines up rather closely with the internal order, this slot was going to be used for a level called "Ocean Wind Zone".
Wood Zone (ID 02)
The level slot containing Wood Zone in the prototypes has something left in the final, albeit less than Hidden Palace: palette, object layout, and music selection data. Accessing it via Pro Action Replay code FFFE10:0200 or Game Genie code ACLA-AECY (AB6X-AEBR with Sonic & Knuckles lock-on) and selecting Emerald Hill Act 1 (or by simply starting the game with the code active) or selecting Special Stage in Knuckles in Sonic the Hedgehog 2) in the level select leads to an objectless (since Wood Zone had no objects to begin with, even the debug object listing only has rings and teleport monitors) version of Emerald Hill Zone Act 1 with Wood Zone's palette, Metropolis Zone's music and some glitchy collision. For some reason, the Zone shows Tails's life counter with no number, and while locked on to Sonic & Knuckles the life counter's text is red. With the aforementioned lock-on, the Wood Zone remains also contain a parallax background scrolling glitch that originally wasn't present with Sonic 2 alone. Some sprites from Metropolis Zone can be loaded into this zone. If you hack in an animal capsule and clear Act 2, you'll go to Metropolis Zone.
Sand Shower Zone (?) (ID 03)
Another empty stage can be accessed with Pro Action Replay code FFFE10:0300 or Game Genie code ACLA-AGD8 (AB6X-AGBR with Sonic & Knuckles lock-on), and like with ID 01 above, it can be accessed by choosing "Sound Test" on the stage select screen (Special Stage with lock-on). This stage is nearly identical to ID 01, with the only differences being that some of the background is still intact, as well as the fact that it uses Oil Ocean Zone's theme as its default music. Badnik sprites from Metropolis Zone and Emerald Hill Zone are loaded. This level slot belonged to Emerald Hill Zone in the Nick Arcade prototype and then went unused in the Simon Wai prototype. Finishing Act 1 will send you to Emerald Hill Zone Act 1.
Going by early concept art which shows roughly what the level order would have originally looked like, and lines up rather closely with the internal order, this slot was going to be used for a level called "Sand Shower Zone".
Hidden Palace Zone (ID 08)
A code or patch so that the 8x8 mapping and metatiles point to OOZ's 8x8 mapping and metatiles.
The level slot containing Hidden Palace Zone in the prototypes actually retains the level's object layout and palette, but the art and block mappings point to Oil Ocean's 16×16 mappings, the level layout points to Oil Ocean Zone Act 1, and the Badniks' code was removed. By using Pro Action Replay code FFFE80:0200 or Game Genie code ACLA-ATD4 (ACLA-ATDR in the "Rev. 00" version of the final, and AB6X-ATBR with Sonic & Knuckles lock-on) and selecting Death Egg in the level select (Special Stage in Knuckles in Sonic 2), you can see this for yourself. This level uses track 10 of the sound test (otherwise unused) and Oil Ocean's debug item list. The second Act can be accessed with the code AGLA-ATD4, and finishing it by placing an animal capsule with debug mode will take you to Oil Ocean Act 1. While locked onto Sonic & Knuckles, the game will automatically reset upon reaching the end of the stage.
Contrary to popular belief, this zone did have minor changes after the Simon Wai and August 21st prototypes. An object that forces Sonic into a ball was added inside the long tube near the start of the level. Another object, intended for the water slide at the end of the zone was added, but does not seem to function correctly. This was absent in all known prototypes. It otherwise seems to be identical to how it was in the Nick Arcade prototype, with the only difference being a lack of badniks, garbled graphics, and almost nowhere to stand on. Decorative objects, platforms, bridges, water, rings, shield, and Tails 1-Up monitors can be found if you use debug mode to navigate the level. Unlike the prototypes, striking this monitor does not grant an extra life, but it does while locked onto Sonic & Knuckles. It is also worth noting that with the aforementioned lock-on, going too far into the stage will automatically reset the game.
Rock World Zone (?) (ID 09)
Another empty level slot, which can be accessed by using Pro Action Replay code FFFE80:0900 or Game Genie code ACLA-AWD8 (AB6X-AWBR with Sonic & Knuckles lock-on) and choosing "Sound Test" (Special Stage with lock-on) in the level select. This version of the stage uses garbled Emerald Hill Zone tiles and badnik sprites from Oil Ocean Zone are loaded, has very little collision data, and no objects present within the debug menu, making it impossible to finish by any means. Just as it is originally the Simon Wai prototype, the default background music is the same as Sky Chase Zone. When finished by hacking in an animal capsule, you'll go to Emerald Hill Act 1.
Going by early concept art which shows roughly what the level order would have originally looked like, and lines up rather closely with the internal order, this slot was going to be used for a level called "Rock World Zone".
Removed Zone (ID 11)
Some of the pointers in Sonic 2 that control things such as zone specific pattern load cues, list another entry after zone 10. Whether or not this is simply an error is unknown, but it is worth noting that 18 zones were originally planned, and that this removed zone would bring the total zone number to 18. Trying to access this zone in the final simply crashes the game on the title card. It's likely this zone was never fully implemented before it was scrapped.
Unused Oil Ocean Zone Background Section
Oil Ocean Zone's background layout is 768 pixels wide internally, but the game only uses the first 512 pixels. Sonic 2 has 64×32 tiles (512×256 pixels) loaded at a time, and loads new tiles on the edges as you move around the level. OOZ's scrolling code doesn't reload any tiles as you move horizontally, leaving you with the same horizontal area of the background that you started with. Presumably, this is because the tile loading would eventually overwrite the tiles used for the sun, which never moves.
|In-game sprite||Correct sprite|
The door sprite in Chemical Plant Zone and Death Egg Zone is actually assembled incorrectly. By using Debug Mode to place a door, you can see how the door's sprite is supposed to be assembled, using 8×8 tiles that go unused due to the incorrect sprite assembly. The reason for this is because the doors use the object subtype $00, the subtype meant to be used by the doors in Hill Top Zone, rather than subtype $02 which is meant to be used in Chemical Plant and Death Egg.
A column of rings found next to a giant cog at coordinates 08F005E8 if you take the path to the right after the first yellow spring shaft in Metropolis Zone Act 2 is very unusual. Each ring collected gives you four rings instead of one, for a total of 12 rings. This is because these rings are actually four columns of rings that are all laid out on top of each other in the same coordinates. This oddity is still there in the 2013 remake.
Unused Level Chunks
Emerald Hill Zone only:
Emerald Hill Zone or Hill Top Zone:
Hill Top Zone only:
Chemical Plant Zone:
Casino Night Zone:
Mystic Cave Zone:
Oil Ocean Zone:
Note that none of the unused chunks are in the table that the game checks to determine whether or not Sonic & Tails should slide.
Wing Fortress Zone:
Unused 2x2 Tiles
Tiles loaded in the VRAM are organized by tileset at RAM address FF9000. IDs are created at this point for each line, representing 2x2 chunks of VRAM graphics. Address FF0000 is in charge of adding mirroring and collision to the 2x2 tiles, creating 8x8 chunks. In all, there are 256 chunks created from FF00000. To add these chunks, add the IDs below in the format 0xxx at FF0000, and search for chunk ID 00 in the stage data. Some unused graphics can be found in this way:
Early Wing Fortress Zone
A small piece of the Wing Fortress Zone appears in Sky Chase Zone, but not all of it is visible because of how the stage scrolls.
This Wing Fortress has several differences from the actual stage version; some windows were removed, others It is possible to note that some windows have been removed and others added, the rivets are placed differently, there is a long metal frame on the left, and a small platform exists near the propellers on the lower right.
There are three revisions of Sonic the Hedgehog 2, known as Rev. 00, 01, and 02.
Region Switch Code: On the title screen, press Up, Down, Down, Down, Up, and a ring sound will be heard. This switches the game between Japanese and International mode, like if the game was put in a Japanese console. In Japanese mode, references to "Tails" are changed to Miles, and the trademark symbols are removed from the SEGA screen and title screen. It can be re-entered to revert it back to International mode. It works the same way on the Japanese hardware (but it's the other way around obviously).
The original release. The following errors are only present in Rev. 00:
|Rev. 00||Rev. 01|
- The signpost in Casino Night Zone is a little too deep into the ground, resulting in the bottom portion of the goalpost being visible through the floor.
- Ring monitors ignore the 999 ring cap, leading to bugged graphics in the ring counter.
|Rev. 00||Rev. 01|
- The credits misspell Thomas Kalinske's name as "Tohmas Kalinske".
- Wing Fortress Zone's checkpoints are bugged: activating one will cause the later ones to appear as if they're activated, too.
- Sonic and Tails briefly display garbled art after activating a Teleport Monitor in 2-player mode. This seems to be the result of both players using the other players' tilemap but applied to their own graphics. Rev. 01 fixes this by making the players invisible until they start moving again.
- The yellow spring walls in Metropolis Zone can be bounced on while hurt, which freezes the character's animation and locks the controls.
- If the big Casino Night Zone springs are landed on while hurt, the character enters a glitched state: they don't enter their rolling state, and remain able to walk, but are still treated as if they're attached to the spring.
The much more common, bugfixed version of the game. Changes in Rev. 01 include:
|Rev. 00||Rev. 01|
- Two flippers in the 2-player version of Casino Night Zone Act 1 changed positions. The Crawl placement in the 2-player version of Casino Night Zone, Act 1 matches Beta 8's placement.
- The flipper and Crawl placement in Act 2 of the 2-player Casino Night Zone matches Beta 8's placement.
|Rev. 00||Rev. 01|
- "Invisible wall" objects are no longer visible in debug mode.
|Rev. 00||Rev. 01|
- The objects that handle the collision for Metropolis Zone's yellow spring walls are no longer visible in debug mode.
This iteration fixes several more bugs, including:
- A crash that would sometimes happen if Sonic defeated a Rexon in Hill Top Zone.
- The ability to transform into Super Sonic at the end of a level, which would get the player stuck in the air (and prevent the level from being completed)
- The background speed and offset variables not being reset if the player continues from a checkpoint in Wing Fortress Zone. This is only a problem if the player gets a Time Over during the WFZ ending sequence.
- The performance is generally better; this is because many superfluous "jump to" commands were removed. As a result, the game only needs to jump to one location.
- Grabber's legs and Mecha Sonic's flame and sparks aren't properly flipped if they're facing right.
- The player cannot Spin Dash off the Tornado in Sky Chase Zone.
- The vertical conveyor belt platforms in Wing Fortress Zone do not make the player fall when they retract into the wall, causing them to walk on air.
Knuckles in Sonic 2
Aside from the obvious addition of Knuckles instead of Sonic, there were some minor changes.
- The game now saves the player's rings after exiting a Special Stage, a la Sonic 3. However, this has the unintended side effect of allowing the player to keep their rings after death if they enter a Special Stage beforehand.
- The bottom two rows of pixels of the screen in Emerald Hill were fixed.
|Sonic the Hedgehog 2||Knuckles in Sonic 2|
- Object layouts were altered, mostly adding monitors and rings in places Knuckles can reach but Sonic and Tails can't.
- The shield and invincibility stars were changed to gray; this is because Sonic uses 4 shades of blue, while Knuckles uses 3 shades of red and 1 shade of green. However, water surfaces in Aquatic Ruin Zone weren't changed for some reason, which makes them look a bit weird.
- All of the cheat codes were changed:
- Level Select: On the title-screen, press Up, Up, Up, Down, Down, Down, Left, Right, Left, Right then hold A and press Start.
- Debug Mode: On the level select, play sounds 01, 09, 09, 04, 01, 00, 01, 08 (October 18th 1994, A.K.A Sonic & Knuckles release date).
- All Emeralds: On the level select, play sounds 01, 06, 07, 07, 07, 02, 01, 06.
- For unknown reasons, all objects except for Rings and Monitors were removed from Debug Mode, with the Monitor now being a Ring instead of Static.
- Path-swapers (which are used by the engine to switch Sonic onto another plane) are now visible in Debug Mode, like they were in some early prototypes. Though unlike early builds these don't play ring sounds
- Two-player mode and the options screen were removed, alongside a playable Tails.
- As a result, all tracks linked to the aforementioned two-player mode (tracks 03, 08, and 0C) are now unused outside of the sound test.