You must beat Team Magma at the "Weather Building". The Magma team is trying to steal a weather Pokemon. After beating them, there's a long wood bridge you must cross. Then you must go up, you will face the boy/girl you met at the beginning of the game (depending on the gender you picked), and you must beat them. Then when you beat them, you go up, (be careful of the fake trees, they have opponents behind them that will face you) at the top. On the left will be berries, the right will contain Fortree City.
Answer
You must travel up Route 119 to get to Fortree City. You will be stopped by Team Magma along the way. You must go to the Weather Center and defeat the leader before you go on.
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Fortree city.
After you get Surf in Pokémon Ruby, you'll have to head to Fortree City which means you'll have to make it back to Mauville City first and then head on your way to Fortree City.
fortree city. flying types
Fortree City ( Feather Badge ) Leader's pokemon: Altaria, Swellow, Skarmory, Pelipper
Use the super rod on the route west of fortree city.