Pokémon Ruby and Sapphire/Fortree City

This is the location of the sixth badge. The city is full of treehouses.

Pokémon Mart
The Pokémon Mart contains the following items:
 * Great Ball: 600
 * Ultra Ball: 1200
 * Super Potion: 700
 * Hyper Potion: 1200
 * Antidote: 100
 * Paralyz Heal: 200
 * Awakening: 250
 * Revive: 1500
 * Super Repel: 500
 * Wood Mail: 50

Go up the ladder next to the Pokémon Center, you'll run into a girl inside the treehouse who wants a Pikachu.

In the next building to the right, an old woman will check your Pokémon for Hidden Power. She'll ask you to guess which hand she holds a coin in. If you choose correctly all three times, she'll give you TM 10 (Hidden Power).

Shop
Continuing down, you'll see a Shop on a tree. The right side sells chairs:


 * Small Chair: 2000
 * Pokémon Chair: 2000
 * Heavy Chair: 2000
 * Ragged Chair: 2000
 * Comfort Chair: 2000
 * Brick Chair: 2000
 * Camp Chair: 2000
 * Hard Chair: 2000

The left side sells desks:


 * Small Desk: 3000
 * Pokémon Desk: 3000
 * Heavy Desk: 6000
 * Ragged Desk: 6000
 * Comfort Desk: 6000
 * Brick Desk: 9000
 * Camp Desk: 9000
 * Hard Desk: 9000

Pokémon Gym #6

 * Prize: Feather Badge
 * Gym Leader: Winona


 * Birdkeeper Jared
 * Doduo: Level 30


 * Picknicker Kylee
 * Swablu: Level 30


 * Camper Terrell
 * Taillow: Level 29
 * Swellow: Level 29


 * Birdkeeper Will
 * Wingull: Level 28
 * Swellow: Level 28
 * Pelipper: Level 28

Skarmory: Level 32 Altaria: Level 33
 * Gym Leader Winona
 * Swellow: Level 31
 * Pelipper: Level 30

Battling Winona
Winona has a mixed bag of birds, so just an Electric or Rock Pokemon can't beat her. Roast Skarmory with a fire type attack. If any of your allies has learnt the Ice Beam attack, then Skarmory and Pelipper will cause problems, as they are not weak against Ice type.

The biggest ordeal is Altaria, whom you'll hate for a lifetime after this battle. It is not weak to electricity. Remember that Altaria usually uses the Dragon Dance move twice before using its deadly Earthquake. So send out a bird at the right time and it will survive the quake. Winona will keep feeding it Hyper Potions to prolong the battle, but she is unlikely to cure any status problems. So, poison or burn Altaria and the Gym will surrender.