Flying Battery Zone
From Sonic Retro
Flying Battery Zone is the second zone in Sonic & Knuckles (the eighth zone in Sonic 3 & Knuckles). It is entered when Sonic or Knuckles latch themselves to Robotnik's flying airship as it passes through Mushroom Hill Zone. Both characters spin through a hatch to leave the airship for Sandopolis Zone.
Sonic 3 Connection
This level was originally meant to be between Carnival Night Zone and IceCap Zone in Sonic 3, with the cannon from Carnival Night shooting Sonic to the flying fortress. Sonic and Tails would have traversed this level normally, but Knuckles would have skipped this level entirely due to teleporting from Carnival Night directly to IceCap. This also explains the lack of Knuckles-only paths in this zone. At the end of the last boss, your character runs down a hallway and hits a panel off the ship. This panel was intended to be the snowboard for Sonic in IceCap Zone. It is believed that the level was moved over to Sonic and Knuckles to balance out the number of full levels in each game (6 Levels Sonic 3, 6 Levels S&K, not counting Hidden Palace and Doomsday, which are actually sublevels.) A custom icon has also been found in S3, so it can be assumed it was very near completion before it got moved. Flying Battery Zone is accessible in the Sonic 3 final version in a corrupted form with a Game Genie. For act one, the code is FFFE10:0400, and for act two the code is FFFE10:0401. For act two, you need to do this in a very specific way because just trying to access the zone alone will freeze the game as soon as the data starts loading. In act one of the corrupted Flying Battery zone, a fully functional Gapsule mini-boss can be found using debug mode.
Flying Battery Zone has three bosses: one in act one, two in act two.
The first boss is an animal capsule with a boss in it. Get on top of the button on its head. When its eyes flash, move off the button and it will hit itself. Repeat this five more times to defeat it.
The second boss is another self-defeating boss, yet an "homage" to a similar boss from a previous game. You just have to avoid the laser as the cage gets smaller and it will eventually defeat itself. You will then be forced to race upward away from an upward-moving platform to avoid being crushed.
The third boss is Eggman swinging on a pair of chains. Avoid being hit by the spiky arms and bursts of flames it shoots, and when the ship starts swinging up and down, use this chance to hit it. It takes eight hits to defeat. An interesting note about this boss is that it is the only time Knuckles fights Dr. Eggman (at every other boss point in his story, he faces the EggRobo instead). This was most likely an accidental oversight with the switch to Sonic & Knuckles.
Instruction Manual Description
Sonic Team Commentary (from Sonic Jam Strategy Guide)