Have you gone down the side passages that branch off at 45 degree angles from Mytos' chamber? One leads to a trapped treasure area, the other skews back and forth and eventually ends up at the doorway to Level 2. Have your thief look for traps, as there are a plenty. Also be on the lookout for a mummy with the Masoleum key on the eastern pathway...you'll need that later.
Good luck!
