Power Castle appears in the Harabah Block of Tartarus, from floors 181 to 189. Their small SP capacity prevents them from using frequent magic skills in battle.
Persona 4
Power Castles appear in B7F and B8F of the Secret Laboratory within the Midnight Channel. They will alternate between single target elemental spells and Mind Charged single target elementals. After being defeated, they may drop the Power Rock material. Selling 8 Power Rocks will unlock the Invincible Mini female armor in the shop for sale.
In Golden, it appears in B3F, B4F, B6F (most commonly) and B7F. It may also drop Rose Quartz for trade at Shiroku Pub.