Bridge Zone

Bridge Zone is the second level of the Sonic the Hedgehog for the Game Gear and Master System. As with all other stages in the game, Bridge Zone consists of two platforming Acts followed by a shorter, third Act which contains the Zone's boss (and nary a single ring). The unusual feature is that Act 2 contains automatic scroll, like Sky Chase Zone in Sonic the Hedgehog 2, which controls the rate at which you move through the level (and makes it impossible to finish in less than 2 minutes).

The zone, as expected, is filled with wooden bridges (some less sturdy than others), as Sonic crosses South Island's massive central river. There are also giant metal scales dotted around the level; presumably these are placed by Eggman as scientific equipment of some sort.

Boss
Considerably more difficult than the first boss, especially if you're playing on the Game Gear's reduced screen (not only is the manoeuvering room reduced, but the fight takes place on sloping bridges rather than level islands). Robotnik takes to the water in an armoured submarine, and shoots trios of energy balls at Sonic as the hedgehog jumps between islands in the river. The trick with this boss is to stand on the central isle and duck. The Doctor's attacks will pass over your head, and then you can hit him before the submersible retreats underwater.

Trivia

 * The background music for Bridge Zone was, years later, remixed as Tails' theme music in Sonic Adventure, "Believe in Myself".