How do you get the dungeon? great game by the way
When you betray the don (by travelling to new ark without paying) all you really have to do is work for him as a "grunt" or something? After that he tells you to find cassie. Once you go to cassie and go to the don again you find cassie there, after that you need to talk to fawn about it, and then the policewoman, and lin lin will be helpful if you dont want an easier time on the first part. After making your way to the don the story plays itself out and you rescue cassie.
To get the dungeon is just renovations from harware r us after this is completed