The Bridge is a type of terrain throughout Nintendo Wars. It is in all games of the Nintendo Wars series.
In all intents and purposes in most games throughout the series, the Bridge is merely around to make the map with Rivers and Sea look better, as it functions in the same way as the Road terrain, giving no defense boost but also having all Movement costs for any land units being 1.
Game Boy Wars 1/2/Turbo
In Game Boy Wars 1/2/Turbo, the Radar Transport Plane can't Load or Unload units when on a Bridge, but instead does so on the Road terrain.
Game Boy Wars 3
In Game Boy Wars 3, the Bridge terrain's practicality differs from that of the Road's, although it has nothing to do with defense bonus or movement cost. Rather, they are important to consider when dealing with terrain-altering equipment.
The Constructor can build Bridges over the River terrain, allowing vehicles with weaker gears to cross and other land units to move through faster.
The Strategic Bomber bombs terrain with Anti-City Bombs. When a Bridge tile is hit by one, it is turned into River terrain, and if a land unit is on it, it is immediately destroyed regardless of HP or Movement Type.