Despite what the name would suggest, this is not a city, but a giant Eggman industrial factory. It features not two, but three huge acts with a massive array of tricky features and traps to negotiate. Portions of the stage loop from top to bottom frequently, and you'll have to be mindful of some nasty industrial crushers, giant nuts and screws, large pits of boiling chemicals and quite possibly the three most irritating badniks you will ever meet. A long and challenging level, but with a great soundtrack.
While Oil Ocean was outside the factory, Metropolis is based solely inside, so the background is basically one long, single wall of machinery. In it, there are panels of different surfaces and colours such as blue/grey, black, red and dark red, plus a wide variety of objects. These background items include pipes, framework, flashing lights, moving compressors, spinning discs, rotating mesh cylinders and much more, so there's always a lot going on back there, even if there isn't much of a landscape around. Most of the foreground is made up of green square and rectangular panels that are bolted down, some with dark green thick diagonal lines across them and come in various sizes.
The road surface is made of similar, thinner strips of bolted down material. There are also large sections of bronze meshing, pipes and tanks, and where they appear along the side walls of your route, you can sometimes find a hidden entrance into them and find power-ups along an invisible surface within. There are also several little multi-coloured details around too, like valves and meters, etc. Like most factory levels of this nature, Metropolis has no foreground decoration along the paths, but this area of appearance doesn't seem too conspicuous in its absence. This level isn't exactly based on cutting-edge technology, and instead, there's a very clockwork theme about it, particularly in the appearance of the objects.
This zone is as massive and long as it is difficult, and with an unconventional three full acts, get ready for a long journey. The structuring is enclosed, meaning that it's made up of rooms and corridors, sometimes fairly spacious, but everything has a ceiling above it, and all the areas are quite well packed in next to each other, making it a bit of a multi-directional maze. Paths are quite varied, with some straight areas, curved ledges and a range of long slopes, some gentle, others unclimbable. There are open spaces where you have to hop across platforms (sometimes above a pit of boiling chemicals), and long vertical shafts containing the giant screws and nuts, or side-mounted springs. This level is a bit of a mixed bag, and seems to have a bit of everything, even fairly short bursts of speed now and then. There is one case where you can fall to your death at the very end, just before the boss, but other than that you're safe in the stable ground. Sometimes power-ups can be found on hidden surfaces within the foreground, so look for rings or items in the scenery around you.
One interesting thing about Metropolis Zone is that it's one of those levels that loops from top to bottom. It's only really noticeable in one point at the end of Act 1 (see below), but in certain places, if you take a route that reaches the top of the map, that route may seamlessly reappear at the bottom, and carry on from there, creating an infinite loop. The multiple routes make use of these with, for example, one route heading upwards, only to mysteriously meet up with a lower one at the bottom of the stage. Very similar to Scrap Brain Zone Act 2 in that way, which many would cite as an inspiration for several aspects of this level. Speaking of multiple routes, Metropolis has a range of them, but only in chunks. There are never two or more completely separate paths through a whole act, or any particularly secret shortcuts to speak of, but some places offer two different ways to go, usually a normal route, and another found by dropping down into an open space. All routes often meet up again not far afterwards, so it's usually fairly hard to determine which one has an advantage over the other. However it's also common for them to split again into sub-routes before rejoining, offering a variety of ways to proceed.
Lots of nasty, very clockwork themed bits and pieces here. Moving tops over pressurised air pipes can spring you upward if they move up while you're standing on them, but at the same time, they release blasts of (strangely harmful) air from either side, so you'll get hurt if you're standing there. Cylindrical tunnels made of meshing rotate continuously, and you'll go around with it, when you step inside, but just run through them to the other end. Watch out for nasty, piston-like thin black crushers that go up and down with force, and there are also more narrow crushers in one particular place in Act 3 (detailed below). Like in a pinball level, some tall vertical passages come with rows of diagonal yellow spring boards along either wall. Touch one and you'll be thrown back and forth between them, up the passage, but be mindful of badniks waiting for you at the top ledges. On one or two occasions, you may have to hold the relevant direction button against the wall to ensure you hit the highest spring and progress.
Small brown square platforms usually appear in groups across a wide open space, and each has a single spike popping in and out of it, one side at a time. It goes clockwise, so first the spike will pop out from the top, then the right, bottom, and left, so you can wisely anticipate where it'll come from next, before you land on the square. Similar to those belonging to Scrap Brain, bronze high-speed transportation tubes are common, either taking you up a brief, twisting pipe or directly to the right, when you step in the entrance, and they sometimes throw you out the other side with speed. They are one-way only. Giant brown cogs rotate constantly but slowly in either direction and are often positioned next to big curved ledges in the road. You can use their steps as platforms to get to a higher level, but don't stay on there too long, or its rotation will knock you off. I don't think you can get crushed between these, so don't be too concerned about that. Hit a nearby button to open or close a brown bridge in the wall, either creating a platform for you across a gap, or opening up an area below, but they switch back again in a few seconds. These can alternatively be found as a long platform resting on a horizontal line of small cogs, which start turning when you land on the platform, moving it across to the right. When it reaches the ledge on the far right side, it'll either stop moving or start heading back to the left when it touches the ledge, so jump off.
And then there are the long grey giant screws, and the nuts that go up and down them. Hop on top of the center of the nut and start running - you'll stay in the same position, but the nut will move along the screw, taking you with it. Run right to go upwards, or left to go down. When playing as Sonic and Tails, you'll find you go much faster if you can get Tails to run on it with you. If he's just standing on the side of it, getting a free ride, stop for a moment and he should then be able to join you in the right part of it. They're usually positioned in long, narrow vertical passages, sometimes with two sets of spikes either side, at the bottom. There are also more difficult sections, involving multiple instances of this feature, and very long distances. Watch out for a lot of Asteron badniks on the walls on either side, because they can be a pain if they happen to knock you off down to the bottom. If that happens and the nut is out of your reach, it will come back down again if you go a reasonable distance away from the area, and then come back. Be warned that you can get crushed between the ceiling (sometimes spiked) and the top of the nut, so don't go up much further than you need to. Sometimes it's easy to get carried away with running on it and not being able to stop in time.
In Acts 2 and 3, you face long pits of boiling, bubbling red chemicals, which are obviously harmful to land in, so be careful when crossing them. To do so, you'll often need to use several series of small platforms which move around a long vertical chain, held up by two rotating circles either end, in a similar idea to a feature of the Labyrinth Zone. A platform will take you up or down, depending on the side it's on, but they may dip into the chemicals as well. Some of the zone's regular brown floating platforms sit on the surface of the chemical pits, but beware, because they drop in when you land on them. Elsewhere, these moving platforms are unusually solid, and unlike most, you can't actually get up on them by jumping up from directly underneath. In a long, narrow shaft, step on one at the top to take you down, like an elevator, and at the tops of vertical passages, two of these platforms will seal off the gap when you pass through from underneath, but won't allow you back down once you're above them. There are also vertical doors in corridors that work in a similar way. Standard conveyor belts are a rare treat, as are groups of nasty singular spikes hiding under long diagonal paths, which will pop up as you pass over them, so be very quick across. In Act 3 only, you can find long platforms moving back and forth slowly, powered by two large clockwork wheels either end. As they rotate, they cause the long platform to slowly move in a circular motion, making leaps onto it or from it only possible at certain positions.
All three badniks that call Metropolis home are among the most annoying you'll ever face. There's Shellcracker, a red crab enemy that shuffles along the ground, with a large protruding claw that it fires out either left or right, when you're standing directly in front of it. Not only is this annoyingly difficult to avoid, he's also hard to kill if you're going for an assault from above, as you have to hit him in an exact spot on his head, else you'll be the one taking the hit. Safest course of action is to perform a spin dash from behind, if you can manage to jump over him successfully, because he's either found in a narrow corridor, or on the edge of a ledge that you're trying to get up to, ready to throw that claw at you as soon as you step onboard. Use good judgement about how far you should jump to avoid him in this case, or try and wait until he's shuffled back a little way first. Upon destruction, his claw and long arm may fly off, possibly causing more problems. Asterons are exploding star badniks that cannot actually be harmed or even touched by you, because they hang out in the foreground walls. When you come too close, they'll move a little bit and start flashing for a moment, before self-destructing, sending the spikes on each of their five points flying out in straight lines. You're sometimes able to run right past them but they're very dangerous in slow-moving sections such as the giant screws, and especially when in groups, as they usually are. When they start flashing, try and get into a position where the spikes will be likely to miss you, when fired out. Finally, Slicer are praying mantis badniks that hang out on floors and ceilings. They'll throw their twin blades as soon as you come rolling along, which will fly up and circle around them, creating a difficult attack to avoid. They have nothing else after that though and can then be easily bopped, at which time the airborne slicers will fall away. You'll almost definitely lose a lot of rings from all three of these guys during your course through the level.