Sonic & Knuckles

Sonic & Knuckles (ソニックナックルズ) is a 1994 side-scrolling platform game developed and published by Sega for the Sega Genesis. An installment in the main Sonic the Hedgehog series, it is a direct sequel to Sonic the Hedgehog 3, which was released earlier that year. The story follows Sonic the Hedgehog and Knuckles the Echidna in their respective quests to save Angel Island; Sonic tries to prevent Dr. Robotnik from relaunching his orbital weapon, the Death Egg, while Knuckles scuffles with Robotnik's minion, known as EggRobo.

Development of Sonic & Knuckles began after the release of Sonic the Hedgehog 2, and was developed simultaneously alongside Sonic 3. The two games were originally intended to be released as a single game; however, due to time constraints and the cost of a large-memory-capacity cartridge, Sega split the project in two. The Sonic & Knuckles cartridge features "lock-on technology" that allows the game to connect to the Sonic the Hedgehog 2 or Sonic the Hedgehog 3 cartridges, combining elements from either game.

Sonic & Knuckles was met with positive reviews. Critics were impressed with the lock-on technology, though some made note of its similarity to its predecessor. It has since been re-released in various compilations and on digital platforms such as Xbox Live Arcade, Virtual Console, and Steam.

Gameplay
Since Sonic & Knuckles and Sonic the Hedgehog 3 were initially developed as one game, their gameplay is similar: both are 2D side-scrolling platformers with similar level design, graphics, and game mechanics. However, in Sonic & Knuckles, unlike in Sonic 3, the player chooses either Sonic or Knuckles at the title screen, Miles "Tails" Prower is not available to select, and the player cannot control two characters together. There is also no multiplayer mode or save feature.

The player character moves through six levels, each divided into two acts. The first act of each level ends with a miniboss fight with one of Dr. Robotnik's robots, while the second ends with a regular boss fight with Robotnik (or EggRobo in Knuckles' campaign). Sonic and Knuckles traverse levels differently: Sonic can jump slightly higher, is faster and can use the insta-shield ability which makes him invincible for a split-second, whereas Knuckles can glide, break obstacles and climb most walls. The levels also include cutscenes that differ based on the character selected, as Sonic and Knuckles are rivals for most of the game.

The game contains two types of bonus stages accessed by passing a checkpoint with at least 20 rings. The first type has Sonic or Knuckles orbit floating, glowing spheres, jetting off each one when a button is pressed, while a fence of light approaches from the bottom and will remove the player from the stage if touched. Collecting 50 rings in this stage earns the player a continue. The second type involves bouncing around a room with a slot machine in its center with the intention of winning extra lives and power-ups.

Special Stages are entered by finding giant rings hidden in secret passageways: the player is placed in a 3D environment and must turn all of a number of blue spheres red by running through them, but must avoid all red spheres, including formerly blue ones. Yellow spheres bounce the player long distances, and white spheres with red stars on them make the player walk backwards in the opposite direction. Completing a Special Stage earns the player a Chaos Emerald; collecting all seven Emeralds allows the player to turn into Super Sonic or Super Knuckles, more powerful versions of the characters.

Lock-on technology
Sonic & Knuckles features "lock-on technology" that allows players to open the hatch on the cartridge and insert a second cartridge. When Sonic 3 is inserted, the player can play through both games as one, Sonic 3 & Knuckles. This features several changes to the games, such as slightly altered level layouts, the ability to play through Sonic 3 levels as Knuckles or Sonic & Knuckles levels as Tails, and the ability to save progress in Sonic & Knuckles levels. Additionally, combining the cartridges is the only way to collect "Super Emeralds", earned by accessing Special Stages in the Sonic & Knuckles levels after collecting all seven Chaos Emeralds from Sonic 3. When all Super Emeralds have been collected, Sonic, Knuckles, and Tails can transform into Hyper Sonic, Hyper Knuckles, and Super Tails respectively, each with unique abilities. Inserting Sonic 2 unlocks Knuckles the Echidna in Sonic the Hedgehog 2, wherein the player can play Sonic 2 using Knuckles' abilities.

If the player attaches any other Genesis game released prior to Sonic & Knuckles, a screen with Sonic, Tails, Knuckles, and Robotnik stating "No Way!" is displayed. From here, the player can also access a minigame based on Sonic 3s and Sonic & Knuckless Chaos Emerald Special Stages. The attached cartridge determines the Special Stage layout. If the player attaches the original Sonic the Hedgehog or Sonic Compilation, the "No Way" screen appears, but the player is able to access all of the possible variations of Special Stages, each with a unique level number and corresponding password. This game is named Blue Sphere in Sonic Mega Collection.

Plot
The story begins immediately after the events of Sonic 3, where Dr. Robotnik's orbital weapon, the Death Egg, is damaged in a battle with Sonic and crash-lands back onto Angel Island. Sonic travels through each zone looking to retrieve the Chaos Emeralds to defeat Robotnik, and once again comes into conflict with Knuckles, who believes Sonic is trying to steal the Emeralds for himself. In Hidden Palace Zone, Sonic fights and defeats Knuckles. The two hear a disturbance outside of the chamber, and go out to find Dr. Robotnik stealing the Master Emerald, the secret to the island's levitation powers. Knuckles attempts to attack Robotnik, but is electrically shocked in the process, and is trapped with Sonic in an underground passage. Knuckles, realizing Sonic is on his side, shows him a portal that leads them to Sky Sanctuary, where the Death Egg is relaunching. Sonic proceeds to infiltrate the Death Egg, and defeats Robotnik as Super Sonic.

Knuckles' story begins in Mushroom Hill, where he is relaxing with his animal friends but is interrupted by a bomb dropped by EggRobo. This prompts Knuckles to chase after him, leading him through most of the same zones Sonic went through. The chase ends at the damaged remains of Sky Sanctuary, where a robot called Mecha Sonic attacks Knuckles but accidentally destroys EggRobo instead. After a short fight, Mecha Sonic uses the power of the Master Emerald to achieve a Super form similar to Sonic's. Knuckles manages to defeat Super Mecha Sonic, who explodes. Sonic flies in piloting the biplane Tornado and Knuckles hitches a ride to return the Master Emerald to Angel Island. If all the Chaos Emeralds are collected, Angel Island rises upwards, into the sky. However, if the player has not collected all the Chaos Emeralds, Knuckles cannot redeem the Master Emerald, and Angel Island plummets into the ocean.

Development
Sonic & Knuckles was developed by the Japanese members of Sega Technical Institute (STI). It and Sonic 3 were originally planned as a single game; due to time constraints and the prohibitively expensive manufacturing costs of a cartridge with more memory, Sega split the game in half, with Sonic & Knuckles as the second part of the Sonic 3 story.

According to Roger Hector, vice president and general manager of STI, the lock-on technology was conceived two and a half years before the release of Sonic & Knuckles, between the releases of Sonic the Hedgehog and Sonic the Hedgehog 2. Early development screenshots suggest Knuckles was planned to be playable in the first Sonic the Hedgehog via Sonic & Knuckles' lock-on technology, but this feature was removed prior to release.

Promotion and release
Prior to the release of Sonic & Knuckles in North America, Blockbuster Video and MTV co-sponsored a tournament contest where children were allowed to play a pre-release of the game, leading to a final tournament held at Alcatraz Island in San Francisco, California, with a winning prize of $25,000 and the title "The World's Most Hardcore Gamer". The two finalists were additionally awarded a copy of every Sega product released for the next year. Shortly before the game's release, MTV broadcast the tournament finale along with other footage of the game as "MTV's Rock the Rock". The game was released worldwide on October 18, 1994.

Re-releases
Sonic & Knuckles has been released as part of numerous compilations of Sega games including Sonic Jam (1997) for the Sega Saturn; Sonic & Knuckles Collection (1997) and Sonic & Garfield Pack (1999) for the PC, Sonic Mega Collection (2002) for the Nintendo GameCube; Sonic Mega Collection Plus (2004) for the PlayStation 2, Xbox, and PC; Sonic's Ultimate Genesis Collection (2009) for the Xbox 360 and PlayStation 3; and Sonic Classic Collection (2010) for the Nintendo DS.

Sonic Jam, in addition to featuring the original release, included some new "remix" options to modify the game: "Normal" mode alters the layout of rings and hazards and "Easy" mode removes certain acts entirely from the game. The version in Sonic's Ultimate Genesis Collection does not retain the lock-on feature included in other versions of the game, because of "tight development times".

The game has been re-released through the Wii's Virtual Console and Xbox 360's Xbox Live Arcade. Both releases are programmed such that, if any of the "Lock-on" games are also downloaded on the same account, the "connected" versions of the game are also available. For example, if one downloads Sonic & Knuckles and Sonic 2, they have the option to play the corresponding Knuckles in Sonic 2 game. When the PC version was released via the Steam software, the games were released together as Sonic 3 & Knuckles as originally intended, with the player (even if playing as Tails) simply continuing at the beginning of Sonic & Knuckles after finishing Sonic 3.

Reception
Critics praised Sonic & Knuckles, despite its similarity to its predecessor. The four reviewers of Electronic Gaming Monthly named it their "Game of the Month". They lauded the lock-on technology and remarked that despite that being "more of the same, it still is an exceptional game." A reviewer of GamePro, who gave it a perfect score, commented that the ability to play as Knuckles makes it essentially two games on a single cartridge, the game is more challenging than Sonic 3, and the ability to hook the cartridge up to Sonic 2 and 3 makes those games "worth playing again." Next Generation called it "the same Sonic game that Sega has sold for the last three years, just wrapped up better and with a prettier ribbon."

Critics praised the lock-on technology the game offered. Lucas Thomas of IGN said it was "a great game on its own", but the lock-on feature completely revamped the overall experience. Dan Whitehead of Eurogamer preferred Sonic & Knuckles to Sonic 3, stating that he could not fully appreciate its predecessor without its "companion piece". Sega Power's review praised the game's harder difficulty in comparison to its predecessor and the new expansion of levels, admitting that the expansion would not have been possible had Sonic 3 been a single game. Sega Magazines review similarly praised the lock-on technology and the new innovation the unique cartridge offered, adding that Sonic & Knuckles hidden stages and bosses would strongly add to the replay value of the combined game.

Reviewing the Virtual Console release, Nintendo Life writer James Newton praised its support for the old lock-on feature of the original release, claiming that the game does not truly shine without having purchased Sonic the Hedgehog 2 and 3 to activate this feature. Thomas praised the game for "impressive visuals that pushed the Genesis to its limits" and for the value added in the content unlocked with the lock-on technology.

The Genesis version sold 1.24 million copies in the United States.

Legacy
For the series' twentieth anniversary in 2011, Sega released Sonic Generations, a game that remade aspects of various past games from the franchise. A remake of the Sky Sanctuary stage was made for the Xbox 360, PlayStation 3, and PC versions of the game, while the Mushroom Hill level was remade for the Nintendo 3DS version of the game. Sonic & Sega All-Stars Racing features a track inspired by the Death Egg as downloadable content along with the Metal Sonic character. The sequel, Sonic & All-Stars Racing Transformed, featured the same race course, and an additional new one inspired by the Sky Sanctuary stage. Re-imagined versions of the Flying Battery and Lava Reef stages, along with the Blue Sphere bonus game, appear in the 2017 game Sonic Mania.