Gremlyn resident Xenor Krell visited the castle, overcoming many dangers on her way to claiming the Ultragem. She was bested by guardian Serpentus but was rescued by the sudden arrival of Amy Rose and Tekno the Canary. Krell tricked them into believing she was retrieving a "stolen" gem for her people, so Amy ventured through the castle and eventually claimed the jewel. Krell and Tekno caught up, the treasure hunter unveiling her villainous side by attacking the heroes. The castle took a beating during the fight, Krell first turning the floor into quicksand before turning the entire castle into a silver palace. This proved to be Krell's downfall, as the castle's polished surface reflected the Ultragem's beam, killing her.
With Serpentus dead and the Ultragem destroyed, Castle Grimm is now just a gleaming monument on Planet Gremlyn and serves no further purpose.