Metropolis Zone is the only zone to have three acts and marks the end of the regular zones. Set inside a huge factory it contains loads of manufacturing processes. Rotating cages, stream engines and nut shafts are some of them. The rapid transport tunnels similar to those seen in Scrap Brain make a return. Some of them have an exit to a shaft of springs ricocheting you around.
From act 2 plenty of mouton metal is around to avoid. Above this is generally loads of blocks with spike sticking out or rotating platforms so the theme here would be to go slow.
Some really nasty Badniks litter this zone. Crabs with punching arms seem to get you all the time as well as the Mantis what will throw blades at you are on the floor and ceiling. Mines populate the walls. There is hardly ever any on their own. They see you and move towards you and then explode sending spikes in all directions. One of them is easily enough avoided but clusters of 5 or 6 make for some trick manoeuvring.
Robotnik will easily make you loose all your rings on this occasion. He'll appear in his pod with balls orbiting him. Should you get a hit on one of the balls it will detach and generate another Robotnik. Hit these and they disappear.
Once you've destroyed all his extras he'll fire a large laser at you from one side of the screen. One more hit will finish him.