From Bulbapedia, the community-driven Pokémon encyclopedia.
Arborville (Japanese: ハテノ村 Hateno Village) is a small treetop village located in a forest Johto that was visited by Ash, Brock, Misty and Sammy in Celebi: Voice of the Forest. According to anime maps, Arborville is located just north of Olivine City. Known residents of Arborville include Towa, Diana, and Mr. White.
It bears a remarkable similarity to Fortree City, a city in Hoenn (first featured in Pokémon Ruby and Sapphire three years later). Just like Fortree City, all of Arborville's houses are built in trees.