Karen's Kingdom is at the South Pole. After defeating Gaito, Karen returns to her kingdom to rebuild it along with the mermaids.
During the short scene where the castle is pictured, only the throne room is shown onscreen. It consists of a throne, multiple columns, and purple ice crystals. The walls and floor are purple as well. Karen's throne is a large clam shell, resembling the Mermaid Princesses' necklaces.