From Bulbapedia, the community-driven Pokémon encyclopedia.
Amber Castle (Japanese: 琥珀の城 Amber Castle) is a small anime-exclusive location in Sinnoh that appeared in An Angry Combeenation!. It appears to be located in the Eterna Forest.
Ash and his friends were trying to find Enchanted Honey with and for Cheryl, with the help of her Mothim.
Amber Castle is behind a waterfall, making it quite hard to find. Inside, it is made up of an abundance of tunnels which lead to a main chamber guarded by a swarm of Combee and a Vespiquen.