Mordred's Fortress is where Mordred build his robot. When you first enter the fortress, Mordred will pose as the princess and will take the mystical weapons of Arturus from you. He will then reveal his true identity, and will enter his throne room. You then must solve a puzzle to enter the room. After that, you will have to fight Mordred in his throne room. After you defeat him and rescue Elyana, Mordred will remain trapped in his fortress until he is sent to Erewhon.