Pokemon currently Native to Imperial Castle:
Druddigon
Croagunk
Toxicroak
Aron
Lairon
Skorupi
Drapion
Sandshrew
Gastly
Haunter
Misdreavus
Pokemon obtainable via special method:
Zekrom (bring Black Stone to this area)
Reshiram (bring White Stone to this area)
Dialga (bring Azelf, Uxie and Adamant Orb to this area)
Palkia (bring Mesprit, Uxie and Lustrous Orb to this area)
Arceus (bring Dialga, Palkia and Giratina in party along with 3 other Legendaries in Party)
Rayquaza (Bring Groudon and Kyogre and have the Green Stone)
Groudon (bring Red Stone)
Kyogre (bring Blue Stone)