How to Get Inside Fortree City

Travel to Mauville City, located west of Route 117 and north of Slateport City.

Exit Mauville City using the east exit to enter Route 118.

Use a Pokemon that knows the move Surf to cross the body of water.

Walk north when you reach the intersection at the end of Route 118 to enter Route 119.

Defeat all of the Team Aqua members at the Weather Institute, located near the end of Route 119 in the large gray building. After the Team Aqua members are defeated, the blockade on the bridge of Route 119 is removed, allowing you to proceed.

Defeat your rival when she appears at the end of Route 119. She has three Pokemon, which vary depending on the type of Pokemon you chose at the beginning of the game, at levels 29 to 31. Use your strongest Pokemon to defeat her.

Continue to follow the path on Route 119 to enter Fortree City. However, you cannot access the Fortree City gym, because it is blocked by an invisible object.

Exit Fortree City from the east exit to enter Route 120.

Head east, south and then west on Route 120 to meet Steven on a bridge. Steven gives you the Devon Scope, an item that allows you to reveal hidden Pokemon.

Return to Fortree City. Use the Devon Scope on the invisible object in front of the gym to reveal a Kecleon. Battle or capture the Kecleon to gain access to the Fortree City gym.