After beating the game, you can stop by the Marvelous Bridge by heading east of Route 16. The previously blocked gate is now accessible, allowing you to go through it.
You can buy a Magikarp from a salesman on the western part of the bridge. It costs $500, and since they can only be found in one other location — the Nature Preserve, which requires you to see all of the Pokemon in the Unova Pokedex (not an easy task) — it's a convenient way to get Magikarp, who evolves into the fearsome Gyarados.
If you haven't yet gotten the Lunar Wing from the Strange House, you may see a ghostly girl while crossing the bridge; if you have gotten it, though, you'll run into the legendary Pokemon, Cresselia, while crossing the bridge!
Cresselia can be annoying to weaken not just due to its high defensive stats and HP, but also due to the fact that it has Moonlight to heal back half of its HP. It has just 5 PP, so it can't use it forever (Pressure or Spite help with that), but it is still annoying. After that, though, it doesn't hit terribly hard, although its low catch rate can make it tricky to catch sometimes.
Anyway, that's pretty much all there is to the bridge. Keep heading across it until you reach your next destination: Route 15!