Sonic & Knuckles

Sonic & Knuckles (ソニック&ナックルズ) is a platform game in the Sonic the Hedgehog series, developed by American studio Sega Technical Institute in collaboration with Sonic Team, and published by Sega for the Sega Genesis/Mega Drive in 1994. It serves as the second half to Sonic the Hedgehog 3. The game features unique "lock-on technology" which enables another cartridge to be plugged in via the socket on top.

Sonic the Hedgehog 4 (both episodes) follows on sometime after the events of Sonic & Knuckles.

Storyline
Sonic & Knuckles carries on from where Sonic the Hedgehog 3 left off. At the end of the Sonic the Hedgehog 3, after Dr. Robotnik's defeat at his Launch Base, the revived Death Egg fell back onto the Floating Island and crashed on top of the Lava Reef's volcanic crater (apparently retconning the ending of the standalone Sonic the Hedgehog 3, in which the Death Egg is vanishes in a flash and is seemingly totally destroyed). The evil doctor resumes his goal of restoring the Death Egg, this time by stealing the Master Emerald to limitlessly fuel its engines. This all-powerful jewel, upon which the other Chaos Emeralds base their powers, is guarded by Knuckles. Dr. Robotnik was able to fool the echidna into working for him and against Sonic. Upon landing in Mushroom Hill, Sonic discovered a Giant Ring that took him to the Master Emerald. Realizing that this is the object of legend that Dr. Robotnik must have set his sights on, he rushes to prevent Robotnik from taking it.

In the underground Hidden Palace, Robotnik takes advantage of the confusion between Sonic and Knuckles in order to obtain the Master Emerald. Knuckles then realizes that Dr. Robotnik has been tricking him all along, and he is injured while trying to stop Robotnik from stealing the Emerald. Knuckles helps Sonic to get to Sky Sanctuary, then leaves them to defeat Robotnik and trusts him to return the Master Emerald. Sonic takes on the Death Egg and defeats Robotnik's escape mech to bring back the Master Emerald, making the Floating Island regain its namesake. However, a lone EggRobo takes up the Doctor's mantle. (Note that Tails accompanies Sonic during his adventure up until The Doomsday Zone in the complete Sonic the Hedgehog 3 & Knuckles - otherwise, he is only seen piloting the Tornado in Sonic's ending.)

A while after this event, Knuckles is seen relaxing with the animals in Mushroom Hill. Suddenly, the remaining EggRobo throws a bomb. Knuckles becomes angry at the EggRobo and chases after him, starting Knuckles's storyline in the game. (Note that this event only takes place in the standalone Sonic & Knuckles - in the full game, he begins at Angel Island Zone without this intro.) As Knuckles races through the zones, Knuckles accesses new places Sonic could not access. After Knuckles exits Hidden Palace through the teleporter, the EggRobo grabs Knuckles and is faced with Mecha Sonic. Knuckles glides out of the way (as Mecha Sonic destroys EggRobo), and the two fight. After defeat, Mecha Sonic dashes to the Master Emerald and uses it to turn into Super Mecha Sonic. Knuckles defeats Mecha Sonic, which makes the platform collapse. However, Sonic arrives with the Tornado, saving Knuckles. Knuckles returns to the Floating Island with the Master Emerald, making it maintain its flight.

Gameplay

 * For a more detailed examination of gameplay, see Sonic the Hedgehog 3.

Gameplays focuses on Sonic and Knuckles; Knuckles was not playable in Sonic the Hedgehog 3 because he antagonized Sonic and Tails. In addition, Tails has been removed from play. Essentially there are two separate games to tackle, depending on which character is selected at the start of the game. Although the appearance of the Zones is the same for either character, their layout and some of the bosses' tricks differs noticeably at a number of points in the game (especially noticeable difference is that Knuckles fights EggRobo, who has different and particularly smarter strategies). Zone differences increase starting from the Lava Reef Zone onwards. Objectives, however, do not differ significantly from one character to the next as they do, for example, in Sonic Adventure.

Despite no save file slots, gameplay is much expanded than in other games, as Sonic and Knuckles do not play all of the same levels. A significant difference between Sonic and Knuckles is that they don't have all of the same moves: Sonic can jump higher and run a little faster than Knuckles, and Sonic has the Insta-Shield, while Knuckles can climb walls and break through some of the blocked entry ways. While Sonic has the gift of speed, Knuckles can glide through the air with the greatest of ease.

Although perhaps not intentionally, none of the zones of Sonic & Knuckles have water, which somewhat reduces the significance of the Bubble Shield, especially for Knuckles, and for Tails in the locked-on game Sonic 3 & Knuckles, as they cannot use any of the shields' abilities which can only be used by Sonic.

As in Sonic the Hedgehog 3, there are two types of Stages: Bonus Stages and Special Stages. The Bonus Stages are entered by collecting at least 20 rings (or at least 35 to access the second Bonus Stage), and jumping through the bright ring of stars that appears above each checkpoint in the Zones. This time however, the gumball stage has been replaced by two new Bonus Stages. The first combines the slot machines of Sonic the Hedgehog 2 's Casino Night Zone and the 360° tumbling labyrinth Special Stage (to retrieve the Chaos Emeralds) of the first Sonic the Hedgehog. If three Robotniks appear on the slot machine, the player loses 100 rings, or all of your rings if you have 100 or less (this does not result in losing a life). The second Bonus Stage is a large vertical course with magnetic orbs that Sonic/Knuckles can use to propel himself upward. Along the way he can pick up shields, rings or lives. Many rings could be collected in this Bonus Stage due to the rising value of the ring gumballs as the player ascends the course - at the very top they can be worth as many as 80 rings. He is chased by a horizontal force field that slowly moves up and when Sonic is caught in it, the Special Stage ends. Reaching the top of the stage ends it as well.

Sonic's abilities
Sonic runs slightly faster and jumps slightly higher than Knuckles, but lacks Knuckles' special abilities to glide and climb. Sonic can use the W spin attack and all the special attacks associated with Barriers.

Knuckles' abilities
Newcomer Knuckles is slower than Sonic and doesn't jump as high, but he can glide through the air, climb up walls, and punch through barriers Sonic would not be able to spin dash through. Knuckles' story is shorter, but it is more difficult than Sonic's.

Zones
The first four zones all have two acts each. As with Sonic 3, there is a mini-boss (or minor bosses, as they are sometimes called) at the end of the first act and Robotnik (or EggRobo) at the end of Act 2. The next two zones each have just one act, and have their own special bosses, as detailed below. Technically, Death Egg Zone has three acts, but there is no screen to say that Act 2 has been cleared.
 * 1) Mushroom Hill Zone: A lush forest, with pump elevators, sticky vines, and filled with mushrooms that also serve as trampolines or parachutes. The miniboss is a woodcutter robot, and for the fight with Robotnik, the player must avoid the spiked bars and cannon shots while hitting the satellite-enhanced Egg Mobile.
 * 2) Flying Battery Zone: The player is propelled to this flying fortress with electrified robots. The mini-boss looks like the Flicky-capsule and must be tricked into hitting itself. Robotnik is fought twice: first, he traps the player in an arena with a moving laser blaster on the ceiling, very similar to the Wing Fortress boss. Then, it goes to an exterior platform which Robotnik latches onto from below and crawls along, spouting flame from the top of the pod every time the character attempts to jump over, and swinging upwards to be hit.
 * 3) Sandopolis Zone: The heroes fall in this desert zone, which houses a haunted pyramid which the character enters after beating the Guardian mini-boss. The boss, Egg Golem, must be hit in the face so Robotnik is exposed.
 * 4) Lava Reef Zone: The interior of a volcano, on which the Death Egg had crash-landed. The miniboss consists of mechanical hands that hit the player and orbs that shoot fireballs. Knuckles doesn't face a boss, but Sonic faces Robotnik in an invincible machine, which is only harmed by the landmines it drops.
 * 1) Hidden Palace Zone: The old Echidna temple. Knuckles just traverses the zone. Sonic and/or Tails however, has to fight Knuckles. After he is defeated, Robotnik steals the Master Emerald to deceive the echidna; Knuckles decides to help the heroes instead.
 * 2) Sky Sanctuary Zone: A secret passage leads to these crumbly aerial ruins. Knuckles just faces his final boss, Mecha Sonic, which uses the Master Emerald to go Super. Sonic and/or Tails has to climb the zone facing Mecha Sonic three times - one in a flail-swinging Egg Mobile, another in a "Flying Eggman", and once on foot.
 * 3) Death Egg Zone: Robotnik's space station, with electric traps, a vacuum chamber and sections with altered gravity. Two minibosses - Red Eye, a vulnerable laser shooting eye surrounded by spiked orbs, and Death Ball, which is only hit when the rotating shield bumpers are away - before facing a Robotnik-shaped mecha, Kyodai Eggman Robo.
 * 4) Doomsday Zone: Only accessible as Super Sonic, this zone has Sonic chasing Robotnik through space, avoiding meteors and missiles, and trying not to get his rings depleted.

Special Stages
The Special Stages in this game are essentially the same as in Sonic 3. There are a few differences though - mainly they are considerably harder than their Sonic 3 counterparts. As well as having the blue, red and grey-and-red bumper spheres, a new kind has been introduced: yellow spheres act as springs and propel the character a set distance across the stage (a distance of five spheres, according to the US manual). Most yellow spheres are helpful, however there are a few that will catapult the player into red spheres, spelling failure for the player.

The Chaos Emerald appears at the end of the Special Stage just like in Sonic 3, although when playing the complete game Sonic 3 & Knuckles this will be a Super Emerald. The same rules apply throughout - enclosing a patch of blue spheres by touching the ones on the outside will turn all those spheres into rings, and collecting all rings in a stage awards an extra life (50,000 points). 50 rings awards a continue.

The Special Stages in the Sonic & Knuckles zones are not quite as straightforward as in Sonic 3. The player will often have to perform clever tricks to complete them, such as making several jumps and turns in succession. As well as being harder in difficulty themselves, there are fewer opportunities to even attempt to get Emeralds. They all have to be collected before the end of Lava Reef Zone, and the Giant Rings can be hard to find.

However, it can be noted that, in the entire first level, 8 Special Rings can be found altogether as Sonic, and 2 more can be found when playing as Knuckles.

Sub-Bosses
The same rules from Sonic the Hedgehog 3. At the end of Act 1, a sub-boss is encountered and needs to be damaged 6 times to destroy it.
 * 1) Hei Hou: Mushroom Hill Zone
 * 2) Gapsule: Flying Battery Zone Sub-boss 1
 * 3) Barrier Eggman: Flying Battery Zone Sub-boss 2
 * 4) Guardian: Sandopolis Zone
 * 5) Heat Arms: Lava Reef Zone
 * 6) Mecha Sonic: Sky Sanctuary Zone
 * 7) Red Eye: Death Egg Zone Sub-boss 1
 * 8) Death Ball: Death Egg Zone Sub-boss 2

Main Bosses
As with Sonic the Hedgehog 3, Robotnik is waiting patiently for Sonic, while the EggRobo usually waits for Knuckles, each in Act 2. Each main boss is required eight hits before exploding into flames.
 * 1) Jet Mobile: Mushroom Hill Zone
 * 2) Hang Mobile: Flying Battery Zone
 * 3) Egg Golem: Sandopolis Zone
 * 4) Hot Mobile: Lava Reef Zone
 * 5) Knuckles the Echidna: Hidden Palace Zone
 * 6) Mecha Sonic: Sky Sanctuary Zone Eggmobile (wrecking ball) rematch
 * 7) Mecha Sonic: Sky Sanctuary Zone Flying Eggman rematch
 * 8) Mecha Sonic: Sky Sanctuary Zone
 * 9) Super Mecha Sonic: Sky Sanctuary Zone (Knuckles' final boss)
 * 10) Kyodai Eggman Robo: Death Egg Zone Final Boss
 * 11) Eggmobile (retreat attempt 2): Death Egg Zone Post-Final Boss retreat
 * 12) The Doomsday Zone Extra Zone and True Final Boss (Sonic)

Lock-on Technology
Sonic & Knuckles utilizes "lock-on technology" that lets the cartridge plus the Genesis/Mega Drive access data from Sonic the Hedgehog 2 and Sonic the Hedgehog 3 so that elements of both Sonic & Knuckles and the locked-on game are combined. To play these combined games, the hatch on top of the Sonic & Knuckles cartridge can be flipped open and the second game placed inside.

The lock-on technology was actually a way of making up for the fact that the developers could not meet the deadline for Sonic the Hedgehog 3. Originally, Sonic the Hedgehog 3 was to be released with Knuckles as a playable character, and with all the Sonic & Knuckles levels playable (the level select screen in Sonic the Hedgehog 3 features inaccessible Sonic & Knuckles levels and music for said levels can be played in the sound test). Time constraints forced them to split the project in two, releasing Sonic the Hedgehog 3 with the completed levels and continuing work on the uncompleted ones, plus adding the Super Emeralds, and the lock-on feature. Indeed Sonic the Hedgehog 2 had once run into similar problems, resulting in the original Hidden Palace and several other levels being removed.

Notably, no other video game in history features a "lock-on" feature quite like Sonic & Knuckles. Some NES games released by HES, as well as the Super Nintendo game Super 3D Noah's Ark, feature a cartridge port on the top to accept other games for the system, but this was purely to override the systems' respective lockout chips, as these games were not licensed by Nintendo. Some video game peripherals require being joined with a cartridge in a similar manner (i.e. Game Genie; GameShark), or transfer data between games (the Transfer Pak) but don't allow the "lock on"-style merging of data from 2 different video games. The technology was considered ahead of its time.

Sonic the Hedgehog 3
Also known as Sonic 3 & Knuckles or Sonic 3 Complete Version, there are eight immediate differences:

Once the player picks a character and starts playing, more differences become apparent. Passing a star post can now open up all 3 Bonus Stages, including the slot machine stage (20-34 rings required), the magnetic orbs stage (35–49 rings required) from Sonic & Knuckles as well as the gumball machine (50–65 rings required) from Sonic the Hedgehog 3. Knuckles' levels are different from Sonic's and Tails' in most cases, with new areas and bosses. There are slight differences in object placement in some of the Sonic 3 Zones as well (Launch Base in particular). Sonic and Tails skips the final boss of Launch Base, while Knuckles still has to do it (although he skips the first true boss of Launch Base). The Sonic & Knuckles levels are the same, except that Mushroom Hill now has an intro (Tails airlifts Sonic in).
 * 1) The player can play as Knuckles in the Sonic the Hedgehog 3 levels.
 * 2) The player can play as Tails in the Sonic & Knuckles levels or have Tails follow Sonic throughout the entire game.
 * 3) After the levels of Sonic the Hedgehog 3 are finished, the screen scrolls downward with the Death Egg rather than playing the normal ending animation, and gameplay immediately proceeds to the levels of Sonic & Knuckles. The final boss of Sonic the Hedgehog 3 is also omitted for Sonic and Tails.
 * 4) Players must collect each Chaos Emerald before their correspondingly colored Super Emeralds can be collected.
 * 5) Game progress can be saved in Sonic & Knuckles using Sonic the Hedgehog 3 's save feature (there are now 8 rather than 6 save slots). In addition, any existing save files from Sonic the Hedgehog 3 will transfer to Sonic the Hedgehog 3 & Knuckles; ie. a game completed in Sonic the Hedgehog 3 will act as a Mushroom Hill save file (this feature is missing in some later compilations).
 * 6) Save slots now display information about the number of continues, lives, and the Chaos Emeralds surround the player's character as the player collects them (the Chaos Emeralds are replaced with their correspondingly colored Super Emeralds when the player collects them). Save files imported from Sonic the Hedgehog 3 will retain the character, last zone and number of Emeralds, but will default to three lives and zero continues.
 * 7) The game uses Sonic & Knuckles ' sound bank exclusively, which means that the songs that were different between Sonic the Hedgehog 3 and Sonic & Knuckles (which include the title screen, 1-Up, and invincibility sounds, as well as the first act boss music, Knuckles' theme and end credits theme) use only the Sonic & Knuckles version.
 * 8) If using 'Debug mode' and a certain number of Super Monitors are destroyed at once, (most effectively with Insta-Shield) then the controls of the character will be locked and by pressing Start the game will turn back into Sonic and Knuckles.

Collecting all the Chaos Emeralds in the levels of Sonic the Hedgehog 3 will turn Sonic or Knuckles into Super Sonic or Super Knuckles. However, once the player enters their first Special Stage in Mushroom Hill Zone, they teleport to Hidden Palace where the Emeralds are taken from them and transformed into gray (apparently depleted of power) Super Emeralds, thus beginning the Super Emerald quest. If the player does not have all of the Chaos Emeralds prior to reaching Mushroom Hill Zone, the acquired Chaos Emeralds will become Super Emeralds and the player is given the opportunity to acquire the first Super Emerald. However, the player will not be allowed to return to Hidden Palace via warp rings until they collect the remaining Chaos Emeralds. If by chance the player has not collected a single Chaos Emerald by Mushroom Hill and goes to Hidden Palace via the first warp ring, the player must exit the area by jumping on the teleporter in the bottom central part of the room. In addition, both Knuckles and Sonic can now go Hyper once they have obtained all of the Super Emeralds.

Tails cannot normally gain Emerald powers but does so when he collects all Super Emeralds, taking on the form of Super Tails. He has no Hyper form but simply remains Super, with four Super Flickies flying around him that attack nearby enemies. Taking care to avoid all the Special Stages, it is possible to finish the game with only the 7 Chaos Emeralds, retaining their Super powers until the very end of the game. In this case the game endings will resemble the good endings from Sonic & Knuckles. This allows for a total of nine different endings — each character (Sonic and Tails together have the same as Sonic alone) has one with 0–6 Chaos Emeralds, one with all 7 Chaos Emeralds and 0-6 Super Emeralds, and the third with all 14 Chaos and Super Emeralds.

Sonic the Hedgehog 2
Also known as Knuckles the Echidna in Sonic the Hedgehog 2 (or Knuckles in Sonic 2 for short, and sometimes called Sonic 2 & Knuckles), this game is identical to Sonic 2 (aside from a few changes to make it more playable with Knuckles) but with Knuckles now playable instead of Sonic or Tails. The title screen features Knuckles and the only option is "Press Start" (Options and the versus mode have been disabled but the level select is intact). There are no new Zones. The only differences in gameplay are that Knuckles can explore once inaccessible areas (where secrets such as 1-ups have been added for Knuckles), some areas are harder because Knuckles cannot jump very high and accelerates slowly (in particular, the final boss is regarded as being much more difficult to beat while playing as Knuckles), the title act cards are green and red and Knuckles retains his rings after a Special Stage or losing a life, making it much easier to obtain Chaos Emeralds. Getting the Emeralds is easier too, for the Special Stages have reduced Ring quotas for Knuckles to achieve, making it much easier to obtain Super Knuckles.

Unlike the Sonic 3 lock-on, where the Sonic & Knuckles ROM referenced the data in Sonic 3 to add its additional features, this game used the Sonic 2 data bank exclusively except for an otherwise hidden extra data bank on the Sonic & Knuckles cart. The lack of knowledge over the existence of this patch made it impossible to find a working dump of the Knuckles in Sonic 2 ROM for a long time.

Sonic the Hedgehog
In the development of Sonic & Knuckles, Sega attempted to implement Knuckles into the original game, but decided to leave him out. According to several programmers and hackers, the reason why Knuckles could not be placed into Sonic the Hedgehog like he was in Sonic 2 was that the palette colors of Knuckles' sprite would have, in fact, completely altered the palette scheme of the entire game. Therefore, instead of a Knuckles in Sonic 1 feature, fans were treated to a full version of the Blue Sphere game. Hackers, however, succeeded several years later in making a game hack called Knuckles the Echidna in Sonic the Hedgehog.The color palette problems are overcome by only using two shades of red for Knuckles' body, rather than four or five. There was also the Spin Dash technique that could abuse the entire game by running at greater speeds than Sonic could.

Any other Genesis/Mega Drive game
By placing the first Sonic game, or most other Genesis/Mega Drive games inside, a "No way? No way!" screen will pop up with Sonic, Tails, Knuckles, and Robotnik making odd faces, telling you the cartridge is supposedly incompatible. But, by pressing the A, B and C buttons together, a minigame based on the Chaos Emerald bonus levels is unlocked, called Blue Sphere. There are over 100 million unique levels. The last level, "Special Stage", repeats itself once the level is beaten. Only by locking the original Sonic the Hedgehog or Sonic Compilation in the Sonic & Knuckles cart can one play through all the stages. The other Genesis/Mega Drive games will only play one level each, which will infinitely repeat. Despite being near randomly generated, the levels are playable for the most part, even if many bits and pieces of levels get recycled. On another note, some levels may be nearly impossible to complete while others are not as difficult.

In Sonic Jam and Sonic Mega Collection (Plus), the Blue Sphere game is playable as a separate game. Sonic Jam allows the player to access it by choosing to play Sonic & Knuckles and "lock it on" with Sonic 1. In Mega Collection, the game has to be unlocked separately. Additionally, there was a PC CD-ROM released, entitled Sonic & Knuckles Collection, that contained Sonic & Knuckles, Sonic 3, as well as the pre-combined version of the two games. This CD-ROM contained a full version of Blue Sphere.

Playing these levels via a game with a battery pack erases saved games. Additionally, games made after Sonic & Knuckles was released will not work with its lock-on feature. There are only a few games made before Sonic & Knuckles that do not work, such as Phantasy Star IV & the Mega Drive/Genesis port of Super Street Fighter II.

The reason that these games are unable to work with Sonic & Knuckles is that the combined size of the two games' data exceeds 4 megabytes, which is the maximum amount of memory that the Genesis/Mega Drive allots for game data.

Each difficulty level contains the icon of all difficulty levels below it. The final difficulty with all the characters present:
 * Nothing
 * Sonic
 * Tails
 * Knuckles
 * Robotnik
 * Mecha Sonic
 * Eggrobo (flying)
 * Chickens
 * Squirrels
 * Rabbits
 * Master Emerald
 * Sonic & Knuckles logo
 * Sonic turns into Super Sonic

Music
The soundtrack for this game was released as the album Sonic & Knuckles Sonic the Hedgehog 3.

Credits

 * Executive Producer: Hayao Nakayama
 * Producer / Lead Programmer: Yuji Naka
 * Director / Lead Game Designer: Hirokazu Yasuhara
 * Senior Game Designers: Hisayoshi Yoshida, Takashi Iizuka
 * Senior Programmers: Takahiro Hamano, Masanobu Yamamoto
 * Character Designer: Takashi Thomas Yuda
 * Scene Artists: Kunitake Aoki, Chie Yoshida, Tsuneko Aoki, Shigeru Okada, Takashi Thomas Yuda, Satoshi Yokokawa
 * Music Composers: Brad Buxer, Bobby Brooks, Darryl Ross, Geoff Grace, Doug Grigsby III, Scirocco, Howard Drossin
 * Sega Sound Team: Bo, Sachio Ogawa, Milpo, Masaru Setsumaru, Tatsuyuki Maeda, Tomonori Sawada, Masayuki Nagao, Jun Senoue

Comic Adaptations
The main storyline of Fleetway's Sonic the Comic in between issues 47 and 53 is loosely based on the game with several changes and plot patterns.

The Archie's Sonic the Hedgehog comics have made an adaptation of this game in Sonic & Knuckles Special, which was second issue of Sonic Special comic books.

Trivia

 * According to the English manual and the Sonic the Hedgehog 4: Episode I website, both Sonic's story and Knuckles' story occur at the same time. However, it is strongly implied in-game that Sonic's story happens first, causing confusion.
 * Strangely, Amy Rose appears in the French guide for the game.
 * Knuckles was intended to face EggRobo in Act Two of each zone, but he ends up facing Robotnik at the end of Flying Battery Zone. This is likely due to the fact that Robotnik's sprite in the Hang Mobile is at a unique angle. Dr. Robotnik also occasionally appears in Mushroom Hill and Sandopolis. On the other hand, if Knuckles manages to cross into Sonic and Tails' route in Marble Garden and Carnival Night, EggRobo will still be Knuckles' opponent despite it not showing up normally.
 * Mushroom Hill returns in the Nintendo 3DS version of Sonic Generations.
 * Sky Sanctuary returns in the console version of Sonic Generations as the third level in said game.
 * This is the first game in the series where Super Sonic is playable during the final boss. This gameplay was later implemented in most other games starting with Sonic Adventure.
 * This is the first title in the classic era of Sonic games to not feature a carnival-themed level.
 * Despite Sonic & Knuckles being considered a direct sequel to Sonic the Hedgehog 3 (continuing literally where it left off), Tails is mysteriously absent until the cutscene after Robotnik is defeated for the final time.
 * However, Tails is playable in the lock-on game Sonic the Hedgehog 3 & Knuckles, the true form of both games, with the full story.