A strategy I learned in my first Caelum mp game: those scouts can conquer any province wihtout provincial defence. I must have taken almost ten provinces in two turns, losing two scouts and a seraphine when some provinces had few defenders... Then I took the rest, got his castle, his sites, slaughtered his sages and did other small things like that. It was fun!
If you do this, your enemy will surely buy some PD. Then you make some raiding parties and kill his PD, which would have slaughtered a scout but can't stand versus 20 Spire Horners! After that, he doesn' know what to do!
And you want to have many fortresses, as between scouts, High Seraphs and Seraphines you just have too many places to produce commanders. Thanks for the Mausoleum hint, Gandalf! Imight actually use them again if that works...