Darkar's fortress was an underground fortress which was difficult to enter, let alone approach, because of the intense negative energy found there, as well as the lack of sunlight, weakens creatures living on the surface and also because it was guarded by the Shadow Monsters of the Under Realm were under Darkar's command. Because of this even Bloom, despite possessing the Dragon's Flame, the most powerful power in the Universe, was weakened when she went to Darkar's Fortress to save the Pixies.