What's this? A flying fortress level so early in the game? Casting a dark shadow over Angel Island, Flying Battery is Robotnik's mighty prison and battleship, and though it has one or two nasty external areas, it mostly takes place inside, where it's actually more of a large, mechanical arena zone. There are an absolute tonne of different traps to overcome and features to use, including the typical end-of-level prison capsules, famously located all over this level and containing various items other than just trapped animals. A mix of speedy, puzzle-based and platform-hopping areas to navigate your way through.
Flying Battery is a huge, heavily armoured military cruiser, but it actually seems to be held afloat by fairly lo-tech propellers and machinery, similar to Wing Fortress, Eggman's previous ship. The sky here is a fine midday blue, and small clouds float by briskly, dipping up and down slightly, as the ship bobs in the air. Most of the time though, you'll be inside, in a large, dark arena, making this more of a mechanical base than a deadly system of air-based structures. A grid of mesh tiles of a very dark blue colour line the background, and they compress in size towards the top and bottom of the screen of Act 1. There are more greenish horizontal beams of varying thickness and shapes going across. Act 2 descends into the ship, where a little more detail is added to this relatively plain and uninteresting background. You can see a row of darkened prison capsules lining one area horizontally and even lower, it seems to get less warehouse-esque and a little more electronic, with various black and green detailed structures, almost subtly resembling microchip architecture. Closer background walls in various small areas carry a basic, but contrasting purple pattern of simple diagonal lines. Though you can see more of the background in Act 2, the general appearance of both acts are pretty much exactly the same.
Inside, you'll be walking on a shiny bronze path, neatly punctured with rows of tiny holes. The internal ground is composed entirely of large square blocks and fuel tanks, both light mauve in colour, and with thick purple chunks of metal lining the walls, with orange edges. The fuel tanks come in various shapes and sizes and are connected by complex piping systems, and usually have little orange windows on them. When you step outside, you'll notice that the interior sections are covered up by the outer shell of the Flying Battery; its massive grey shutters, camouflaging the ship with a military/combat pattern all over it. The sturdy pathways out here are more of an impressive bronze structure with small round windows and bolts, very similar to that of Wing Fortress. The thinner, structural walkways that can be mostly found outside are based on less solid frameworks. Like most predominantly mechanical zones such as this, Flying Battery contains no additional decoration or props on the ground, and though its background is pretty boring in most areas, the surrounding ground offers plenty of colour and variety to make up for it.
Both acts are very enclosed, with most of the action taking place in a series of rooms and corridors within the fortress. In Act 1, many of these internal chambers are fairly large and spacious, but less so in Act 2, which relies more heavily on its network of passageways. Flying Battery has a good dose of speed, with some loop-de-loops connecting narrow vertical tunnels built into the ground, and large rotating cylinders in which to spin around and tumble out the other end. Though there are some curved roads and ceilings, most of the pathways are relatively straight, with only gentle slopes, as a rule. Slower areas can be more puzzle-based, forcing you to wait until the right time to make your move, and not get hit or crushed by an obstacle. Exterior sections are surprisingly minimal, but when outside on the bottom of the ship (as you will be early on in Act 1), you'll have to hop across a series of floating platforms, and death awaits any spined mammal careless enough to slip and fall off one of them. You'll be winding your way back and forth in all directions in this long, large level.
Sonic and Knuckles take the exact same main route across the zone, and start and end at the same places. At first glance, you may think that this is a straight-forward, single-route stage, but there are actually several well hidden alternate routes on offer for those that love to explore, each of which snaking its way covertly across the map, linking two otherwise disparate points. There are three shortcuts in Act 1, all beginning at different points around the middle, and that take you away from the main route. Only Knuckles (and Tails in S3&K) can access the first, and most efficient one which takes you up to the top of the stage, and skips out a huge portion below. These hidden routes are explained in Points 3, 4 and 6. Act 2 doesn't disappoint either. Choosing your exit from an early spiral elevator gives you the option of two separate routes to take on the first portion of the act, one much harder than the other, so choose wisely at Point #8. Soon after these join, two more, somewhat well hidden alternate routes emerge from the main one, all meeting at the same area towards the end, and taking roughly the same amount of time to get there too, although with varying degrees of difficulty and fun! See Points 9 and 10 for more details.
Get ready to interact with one hell of a lot of different obstacles here. All moving platforms have a large fan underneath to keep them afloat, though touching the fan actually causes damage, so stay on the nice flat bit at the top. They can travel in all directions, though some only move when you step on them, then remain at their destination. Others don't really go anywhere at all. When you see a series of white rectangular monkey bars on the ceiling, jump to hang from them, and climb across them using the left and right buttons. One or two handles in the series may drop you down to a lower level when you grab hold of it, so you often have to find one of these in order to progress. Watch out for flame throwers within short grey cylinders on the ground. Most of them have a lick of flame either side of them, each of which flails wildly as the cylinder rotates. A few just have one flame on a single side only, and many can be double stacked in one cylinder. If there's a flat top on one, jump on it and stand there for a moment and it'll act as a spring for you. In one instance, one of these actually act as a button to open up a gate. Such gates, coming in horizontal or vertical flavours, are more commonly opened by a plain old regular grey button next to them, but they do appear an awful lot, especially in Act 2.
The long horizontal spinning mesh cylinders from Metropolis make an encore appearance in this level, this time in more diverse formations. Step in one and you'll spin around inside it, though make sure you're in the highest position necessary when you reach the end, in order to land on the next platform or another cylinder. Larger, vertical ones work in a similar way, with the character leading downwards as they enter from the top left to the bottom right, except they're not always spinning and if you don't run fast enough you'll just fall straight through it.
Small, solid orange and blue circles feature in a variety of traps and tricky situations, and you often need to be careful about getting crushed between them and other solid objects. At a couple of points in Act 1, strings of them will travel around in square formations, using an arrangement of tube bends to change their directions. You need to use these platforms to get up to a high ledge, but don't get caught between the circles and the bends! See Points 2 and 5 for more details. Other times, lines of them will closely orbit one large circular object, also solid, so you'd need to time your movements so that you can use the whole collection of objects as a platform, without getting pushed off by the small circles as they come around.
One of Flying Battery's most well-known traits is the prison capsule - exactly like the regular animal capsules that you open at the end of every level, but they can be found all over this stage. As usual, hit the yellow button at the top to open them, in order to free either a bunch of animals, rings or even the booby prize - badniks! Others can't be opened, and instead the button acts as a spring, but there's no visual cue as to what is in each one. It's never random though, so you just have to learn them on a case-by-case basis. Outside, at the top of the ship, you'll find long, spinning bronze poles that you can grab on and spin around with. Use the up and down buttons to move along it, and then jump to release your grip and fly forward. On them are propellers that harm you, so leap from one pole to the other carefully, and also beware of missiles that shoot out of the walls at you. Use small speed-up launchers on the floor for a quick horizontal boost and be careful of small grey mines, arranged along a section of the ground. If you step on one of them, it'll explode in a small puff of air after a brief moment, but it's enough to do you some damage. Often leading up to shortcuts, look out for small black fan ledges that span out from the walls of a vertical passageway, leading upwards. Use them to leap from side to side, though be quick before they disappear.
After a certain point in Act 1, you'll encounter large metal bars on the ceiling that produce electricity inside them at regular intervals. These last for a few seconds at a time, and causes the bar to be magnetised, which pulls up the heavy metal blocks that lie underneath it, attached to the ground by chains. When in corridors, the blocks make contact with the bottom of the bar and they can either crush you there, or between the ground, when they fall back down afterwards, and you can also find large spikeballs resting on the ground that work in the same way. When in an open area, the large blocks on chains only go up to a certain point, as much as the chain will allow, so you can actually use them as platforms there, to get to a higher ledge. On the ground between the blocks and in many other places you'll often find rows of spikes - special Flying Battery brand spike sets that is, in which each of the spikes sort of jut up and down a little bit. This is, perhaps, only to quietly suggest an alternate property that a couple of specific ones in Act 1 possess - you can actually push them! See the Special Stage rings section of Act 1 to find out how you can push them aside in order to access two different big rings, hidden below them!
On two occasions (one being a shortcut) on the upper outside of the ship in Act 1, missiles shoot out vertically in threes from square hatches in the ground. They then drop down by parachute, landing and exploding on certain areas of the floor. Dodge these and go to the far right end of either of these two sections, where, after taking six missile hits, a small part of the floor will open up and allow you to drop through. To be safe from them, stand directly above the missile hatch.
In Act 2, you'll encounter another type of crusher - a continuous stream of mauve spiral platforms slowly move up or down a long rotating orange cylinder. Step on one of the gently sloping platforms to go along with it, but get off quickly at the exit, because these things will crush you into the ground or ceiling without hesitation. These are always found in narrow vertical passages, with small spikes lining the walls, and you frequently have to press a button to open a gate to them. On a couple of rare occasions, you may start out on one side of the corridor, wanting to get directly to the other side, but one of these things is in the way, moving up into the ceiling so you have to get through it very quickly so that you don't get crushed. Timing is crucial there, especially given their awkward shape.
If you remember the Grabber badnik from the Chemical Plant Zone, you'll recognise a similar thing here, only it's not a badnik, it's a harmless bronze contraption that drops down from the ceiling and grabs you. Let it, and it'll pull you back up and take you horizontally across a rotating vertical mesh cylinder (which you can't cross by yourself at that point) and drop you off safely at the opposite ledge. In speedy sections, use a line of black orbs, mounted in the middle of a ledge, to take you in a semi-circular direction from one side to the other. Probably the funnest thing in the level! There are a couple of collapsing portions of some floors, dropping you in on a separate route rather than causing any particular inconvenience, and some very narrow vertical gaps in the ground are obstructed at the top end by a block that can be broken by jumping on it. Other examples of these narrow tubes are designed to be sprung up through from below, and there may be one or two red springs inside mounted on flippable circular hatches. These spin as you pass them, allowing the spring on them to propel you further up.
After all that, Flying Battery thankfully only features two badniks. Technosqueak is a mouse badnik (resembling a mixture of the mouse animal, and a computer's mouse), that scuttles quickly back and forth across a set area, either on the floor or ceiling. Blasters, on the other hand, trudge around very slowly. They're tank robots equipped with a large, diagonally mounted cannon which they use to shoot out gravity-prone projectiles. While underneath the long electrical bars, they too can be magnetically drawn to them, where they'll be easy to defeat.
- Flying Battery is the second of Robotnik's grand flying fortresses over the series, well armoured and loaded with weapons but also seemingly designed to transport imprisoned animals from A to B. It also serves as another trap for Sonic, while the doctor is desperately throwing everything he has at him, in order to once again relaunch the Death Egg. It's unclear whether or not, storyline-wise, Sonic actually defeats Flying Battery, causing it to crash, and if so, how. It's probable that Eggman caused the explosions himself, during the pre-boss laser trap. Then again, if Knuckles' game is supposed to take place after Sonic's (as there is a good amount of evidence to suggest so), that means the ship must have survived, or would have at least been revived for it.
- In the Act 2 boss, Knuckles will fight Eggman rather than Eggrobo. This is the only time throughout Sonic 3 and Sonic & Knuckles that Eggman even appears in Knuckles' game, so why here? Why not as part of the final bosses, or something? I've always had my suspicions that it was a mistake on the developers part, and it was supposed to be Eggrobo all along. Consider though that Eggman's sprite is facing front-on in this boss, and as far as I'm aware, Eggrobo has no front-on sprite. Perhaps someone was just too lazy to make one? All rather strange to me.
- Flying Battery was originally designed to be a Sonic 3 level, linking Carnival Night to Icecap. If you access the hidden sound test menu in Sonic 3, which also contains a level select, you'll find its name in the list, but you can't access the level.