Morgause's Fortress

We don't know where the Morgause's enchanted Fortress is located. Guided by Arthur's horse, Arthur and Merlin had to cross a lake and a waterfall to reach it. It was in a courtyard of the Castle that Morgause challenged Arthur. She asked him to put his head on the block and pretended to let the axe she was holding falling on his neck. She tested if Arthur was truly a man of his word and to reward his courage and honesty, she granted him to see and talk to his mother. Then he learn the truth about his birth.