Sonic & Knuckles

Sonic & Knuckles is a platform game in the Sonic the Hedgehog series series, released for the Sega Mega Drive/Sega Genesis in 1994. It is the direct sequel to Sonic the Hedgehog 3, which was released earlier that year. The game features unique "lock-on technology" which enables another cartridge to be plugged in via the socket on top.

Storyline
Sonic & Knuckles carries on from where Sonic the Hedgehog 3 left off. At the end of Sonic the Hedgehog 3, after Dr. Robotnik's defeat at Launch Base Zone, the rebuilt Death Egg fell back to Angel Island and crashed on top of Lava Reef Zone's volcano. Now the evil doctor attempts once again to repair the Death Egg, this time by trying to use the Master Emerald to fuel its engines. This all-powerful jewel, upon which the Chaos Emeralds base their powers, is guarded by Knuckles. In Sonic the Hedgehog 3, Dr. Robotnik was able to fool the echidna into working for him and against Sonic. In Sonic & Knuckles he betrays Knuckles in order to obtain the Master Emerald. Sonic and Knuckles then work together in order to stop Robotnik and save Angel Island. In this way Sonic & Knuckles resolves the feud between the two characters, and ties up the plot of Sonic the Hedgehog 3. Playing as Sonic is the first part of Sonic & Knuckles, and playing as Knuckles is the second half.

Knuckles' adventure is part of a separate storyline that takes place shortly after the Death Egg is destroyed in Sonic's adventure. This is evidenced by the following points:
 * The backgrounds of Angel Island Zone and Sky Sanctuary Zone show Angel Island floating in the sky.
 * The image of the Death Egg has been removed from Launch Base Zone, Lava Reef Zone and Hidden Palace Zone.
 * Sonic's ending concludes with one of Robotnik's EggRobo Badniks emerging from the wreckage of the Death Egg. It's implied that this is the same EggRobo that drops a bomb in Knuckles' intro scene and pilots the boss vehicles in his adventure.
 * The ghosts have already been released in Sandopolis Zone.
 * In Angel Island Zone, Carnival Night Zone, and Launch Base Zone the water has been drained due to the fact that Angel Island is now floating in the sky.

Gameplay
This time the game focuses on Sonic and Knuckles, who was not yet playable in Sonic the Hedgehog 3. Instead however, Tails has now 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, increasing from the Lava Reef Zone onwards. However, the style of play does not differ significantly from one character to the next as it does, for example, in Sonic Adventure.

The reason for any differences in storyline or gameplay in Sonic and Knuckles's routes relate to the time of the events taking place. Knuckles' route occurs after Sonic's, and after the Death Egg has already been destroyed; this is why Robotnik and the Death Egg are not seen (or at least not intended to be seen; there is a graphical "bug" in the Flying Battery's final boss) in Knuckles's route.

Levels
Sonic & Knuckles features 8 zones in total, but depending on the selected character and whether or not the quest for the Chaos Emeralds was completed, the game may be finished in 6 or 7 Zones instead.


 * 1) Mushroom Hill Zone: An autumnal landscape covered with fantastic mushrooms.
 * 2) Flying Battery Zone: This flying fortress is a high voltage engine littered with loose screws and force fields, reminiscent of Sonic the Hedgehog 2's Wing Fortress.
 * 3) Sandopolis Zone: A desert temple with shifting sands. Act 2 takes the player into the dark and haunted tomb of a long forgotten pharaoh, within which malicious ghosts dwell.
 * 4) Lava Reef Zone: Lava Reef is the biggest and hottest volcano on Floating Island. Dr. Robotnik's Death Egg machine is located somewhere in this Volcanic crater.
 * 5) Hidden Palace Zone: This ancient cave is where the mysterious Super Emeralds are hidden. For Sonic, crucial plot points are revealed here. For Knuckles, Hidden Palace is merely a passage to Sky Sanctuary.
 * 6) Sky Sanctuary Zone: The battle takes to the clouds as Sonic fights Dr. Robotnik's badniks across the crumbling ruins of an ancient civilization. Sky Sanctuary is the end of the game for Knuckles.
 * 7) Death Egg Zone: Sonic has now breached deep into enemy territory, and finds himself in the heart of Dr. Robotnik's giant space station. Death Egg Zone is only playable with Sonic (or Tails when locked on with Sonic 3). Unless all the Chaos Emeralds were collected, this is where the game ends with Sonic.
 * 8) The Doomsday Zone: In this final zone, Sonic unleashes incredible Emerald forces in his ultimate showdown with Dr. Robotnik. The Doomsday Zone is only accessible if the player has collected all the Chaos Emeralds. Completing it unlocks the "good" ending. Here, the player uses Super Sonic rather than normal Sonic &mdash; or if locked-on to Sonic 3 and the player has collected the Super Emeralds as well, the player can play as Hyper Sonic.

Special stages
As in Sonic the Hedgehog 3, there are two types of special stages: bonus stages and emerald stages. The bonus stages are entered by collecting at least 20 rings and jumping through the bright ring of stars that appears above each lamppost 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 of Sonic the Hedgehog. If Sonic hits a "goal" block the stage is over.

The second bonus stage is a large vertical course, with magnetic orbs that Sonic can use to propel himself upward. Along the way he can pick up shields, rings or lives. He is chased by a horizontal force field that slowly moves up and, when Sonic is caught in it, ends the stage, and reaching the top of the stage ends it as well.

The emerald stages have remained nearly unchanged from Sonic the Hedgehog 3, the only differences being the layout of the individual stages and the addition of a yellow sphere that acts like a spring.

Lock-on Technology
Sonic & Knuckles utilizes "lock-on technology" that lets it 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 of the Sega Technical Institute 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" extra bonus games, and the lock-on gimmick. Indeed Sonic the Hedgehog 2 had once run into similar problems, resulting in the original Hidden Palace and several other levels being removed.

Sonic the Hedgehog 3 with Sonic & Knuckles
Also known as Sonic 3 & Knuckles (often referred to as the "complete" version of Sonic the Hedgehog 3 by Sonic fans). There are four immediate differences:
 * 1) The player can play as Knuckles in the Sonic the Hedgehog 3 levels.
 * 2) After the levels of Sonic the Hedgehog 3 are finished, play continues straight to the levels of  Sonic & Knuckles
 * 3) The player can play as Miles "Tails" Prower in the Sonic & Knuckles levels or have Tails follow Sonic throughout the game.
 * 4) 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).

Once the player picks a character and starts playing, more differences become apparent. Passing a star post can now open up 3 bonus games, including the slot machine stage (20 rings required) and 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. Sonic and Tails skip 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. Sonic 3 & Knuckles save files conserve lives and continues, as well. In addition, Knuckles' theme music has also changed as well as that of the mid-level bosses. Other tunes such as the 1-Up fanfare and title screen have also been changed to the Sonic & Knuckles version. Monitor (power up) placement and type change when the games are combined.

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 level in Mushroom Hill Zone, they teleport to Hidden Palace where the emeralds are transformed into gray "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. In addition, both Knuckles and Sonic can now power-up and go "hyper" once they have obtained both the Chaos Emeralds and 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 some extra powers. Taking care to avoid all the Special levels, 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 with Sonic & Knuckles
Also known as Knuckles the Echidna in Sonic the Hedgehog 2, this game is identical to Sonic the Hedgehog 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 "start" (Options or versus mode have been disabled). 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), and Knuckles retains his rings after a Special Stage, 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.

Unlike the Sonic the Hedgehog 3 lock-on, where the Sonic & Knuckles ROM referenced the data in Sonic the Hedgehog 3 to add its additional features, this game used the Sonic the Hedgehog 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 the Hedgehog 2 ROM for a long time.

Any other Mega Drive game with Sonic & Knuckles
By placing the first Sonic the Hedgehog game, or most other Sega 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 134,217,728 of these levels. Only by locking the original Sonic the Hedgehog game (the Sonic Compilation works as well) in the Sonic & Knuckles cart can one play through all the stages. The other Genesis 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.

In Sonic Jam and Sonic Mega Collection, Blue Sphere is playable as a separate game. Sonic Jam allows you to play it by choosing to play Sonic & Knuckles and then choosing to "lock it on" with Sonic 1. In Mega Collection, the game has to be unlocked separately.

It was long suspected that the reason that Sonic the Hedgehog used a different lock-on game was that if Knuckles were to glide on the conveyor belts on some of the zones, that the game would crash. This is almost certainly a hoax, though of course this cannot be directly proven. However, because many enemies and much of the level art used part of the palette line reserved for Sonic, changing it would make some of the graphics become messed up. This was not the case in the other two games, where Sonic's blue palette entries could not be used for other art because it would be replaced with the Super Sonic scrolling palette, leading to corrupted graphics again. As a result, by copying Knuckles' colors over the Sonic palette line in Sonic 2 or 3, the graphics are not corrupted.

In September 11, 2005, a Sonic game modifier named Stealth created a [ROM hack that allowed you to play Knuckles in Sonic the Hedgehog called Knuckles the Echidna in Sonic the Hedgehog, with all of Knuckles' unique abilities intact.

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 reason that these games are unable to work with Sonic and Knuckles is that the combined size of the two games' data exceeds 4 MB, which is the maximum amount of memory that the Genesis allots for game data.

Trivia
Lava Reef has 4 Acts, The "true" Acts (Act 1 and Act 2), Robotnik battle (Act 3), and the Sonic 3 & Knuckles Hidden Palace Zone (Act 4).
 * Sonic & Knuckles was one of the first Sega Genesis games to be packaged in cardboard when it was initially released. Most Genesis games were released in plastic cases, but later in the console's life, cheaper cardboard boxes began to replace them.
 * Sonic & Knuckles was the first Sonic game to offer an all Super Sonic level. With the exception of Chaos in Sonic Adventure and the Last Story (Metal Overlord) in Sonic Heroes, all the Super Sonic levels/bosses have been in space. These include Doomsday Zone, Finalhazard (Sonic Adventure 2), Moon Zone (Sonic Advance), True Area 53 (Sonic Advance 2), Nonaggression (Sonic Advance 3) and Exception (Sonic Rush).
 * Sonic & Knuckles was the first and only video game to use lock-on technology to combine elements of two games. Some unlicensed SNES games used a superficially similar arrangement to bypass the SNES's lockout.
 * The background during the fight between Sonic and Knuckles in Hidden Palace Zone is actually a reference to the last level (Doomsday Zone). It is a picture of Super Sonic fighting Robotnik, made to look like an ancient artwork. This idea is reused in Sonic Adventure.
 * Knuckles in Sonic the Hedgehog 2s debug code in the secret Level Select Menu is 1,9,9,4,1,0,1,8 or 10/18/1994- the release date of Sonic & Knuckles.
 * In the level select screen accessible by a code at the title screen, Hidden Palace does not appear, instead indicated in the menu as Lava Reef 4. Likewise, the Lava Reef Act 2 boss is actually coded under Lava Reef 3.
 * The Archie Sonic Comic Special features a story loosely based on the game.
 * The designers surveyed many players to determine whether or not to reverse the "Duck" and "look up" controls in the Anti-Gravity sections of the Death Egg. Many players found it more natural to press down to crouch even when the scene flipped so the basic control was retained.
 * For some unknown reason, Knuckles encounters Robotnik as the final boss of Flying Battery Act 2 instead of the EggRobo. The possible reason is that Robotnik's sprite in this battle has a different appearence than in all of the other fights, and the designers either forgot or never bothered to adapt it into EggRobo.
 * There is a glitch when fighting the boss in Mushroom Hill Act 2 where EggRobo turns into Dr. Robotnik.