Final Zone

Final Zone is the seventh and last Zone in the original 16-bit version of Sonic the Hedgehog. Final Zone is the shortest zone in the game, as it only features the final boss of the game, while having the final fight against Dr. Robotnik inside his private laboratory.

Sonic must defeat Robotnik without getting hit, as there are no Rings to protect Sonic, which makes it one of the toughest bosses in the game. When the player starts at the beginning of the Final Zone, they will have to to move towards the final room to start the final battle against Dr. Robotnik, who uses four engine pistons to defend himself and crush the hedgehog.

Boss guide
When Sonic enters the laboratory room, Dr. Robotnik attacks him by using four large engine pistons. These pistons are able to easily crush Sonic into the ceiling or floor.

Every time two of four pistons come out; one decoy, and one with Dr. Robotnik laughing inside of it. Sonic must attack the piston with Dr. Robotnik in it. Each time the pistons move, Sonic is able to attack Dr. Robotnik once. However, the player has to be careful depending which pistons will come out attempting to crush Sonic. Also, between each piston cycle, there are also small electric generator creating four spark balls, that come from the ceiling towards Sonic, who must dodge them. When the generator is creating and targeting these spark balls, the player has to choose a place where to avoid them. Due a lack of rings, touching a spark ball results in instantly losing a life. The player can target them to the center of room and then do a Spin Jump to dodge them completely. After this, another two pistons will appear, and the battle keeps repeating the pattern.

Like every other boss in the game, Sonic must hit Dr. Robotnik eight times to finish off the doctor for good. After landing all eight hits, the pistons start exploding and Dr. Robotnik escapes from the room. Sonic must follow him, as Dr. Robotnik hops into his Egg Mobile (in its rare landing state) and starts flying away. During this time, the player has two choices, but his or her decision does not greatly affect the outcome of the game. If the player lets Sonic do his finishing move on the Egg Mobile, it starts exploding and is wrecked, slowly falling down into the bottomless pit at the end of the zone. The player can also simply let Dr. Robotnik go.

After clearing the Final Zone, the game progresses to the ending sequence and credits. The ending sequence differs based on whether or not Sonic has collected all of the Chaos Emeralds from the Special Stages.

Sonic X (comic series)
The Final Zone (mistakenly told to be Final Stage) is briefly featured in Archie Comics's Sonic X #11. After Sonic saved Cream, Cheese and Vanilla from the glass box, Cheese pressed the switch, that transported the whole gang to the Final Zone, where Eggman is in one of pistons was waiting for them.

When the gang start to finish Eggman and his pistons, Eggman outside of the video game, had almost getting enough points to launch Sonic and his friends to the moon, who actually were trapped to the tower base. At the same time, Chris and Chuck managed to release Sonic and friends from the game, that and the game was powered by Master Emerald before the base tower got destroyed.

Archie Comics
The Final Zone is also featured in the Archie Comics's Sonic the Hedgehog #227 that takes place of "Genesis" arc. When Sonic, Sally and Rotor entered the depths of the Scrap Brain Zone, Sonic suddenly got a magic forcefield around him and Dr. Eggman showed up inside one of pistons. After talking, both of them started fighting against each other, until Sonic found himself caught in a dead end.

Before Eggman managed to crush Sonic, water suddenly started popping out above the piston where Eggman is in it with several pieces of Orbinauts from Labyrinth Zone, and Antoine (who before handed fell down to the endless hole at the Labyrinth Zone) found himself on Doctor's arms, and, at panic, he smacked Eggman to the face. This gave Sonic the chance to finish Eggman and his pistons for good. While Freedom Fighters tried to find Eggman, who was seen silently escaping in the background. Sonic noticed this and tried to catch him, but unsuccessfully. While he managed to hit him on his Egg Mobile and made him crash, Sonic believed that this was not the end of the doctor.

Trivia

 * Final Zone was originally conceived of not as a single zone, but rather as full stage-length Act 3 of the Scrap Brain Zone.
 * In the 2013 re-release of original Sonic the Hedgehog, Final Zone has only slight changes in normal in-game, as the doorway, where the player comes to the room, will be closed for the battle, as the tube at the other side of room will be solid until it breaks off after defeating the final boss. These changes have been made due to the widescreen feature of the re-release.
 * If Sonic delivers the final hit to the boss and hits twice instead (with the right timing), a glitch will happen which indicates that the boss needs to be hit another 255 times.
 * After defeating the boss, a glitch may happen that causes Sonic to jump into a bottomless pit, thus losing a life.