This was back in 3.204. As Marshal for the Empire I had:
Killed the Sarleon and D'Shar
the Empire controlled 3/4 of the map
I personally controlled 11 holdings - including 4 cities
Very high relations with dozens of Lords
12 Right to Rule.
This route was a lot of fun, but I stopped because there seemed no way for me to win. There was no way to overthrow Marius and take control of the Empire. And if I renounced my Oath to start my own Kingdom, I lost all my holdings and it would be virtually impossible to beat the Empire I had built up.
Am I missing something that there's no way to win from a Vassal or Marshall where you can take your holdings with you, use the lords that love you, and start a kingdom and win?