Don't know for sure if this is correct, however, the sixth circle was for heretics. According to different legends about Medusa: 1. she was either born with the snake hair, 2. she was a beautiful woman who bragged she was more beautiful than a goddess (or all goddesses) - a form of disrespect/heresy, or 3. she was a priestess of Athena and conducted an affair with a god (Poseiden?) in Athena's temple - another form of disrespect to the gods, in this case Athena.
Depending on how you read it, The City of Dis would not be circle 6, rather, the gate or entrance to the Capitol of Hell. She wasnt truly considered a Heretic, rather a figure who makes her own rules. A tormentor of souls if you will, an infernal fury.