After several campaigns (huge units setting, normal difficulty), I advocate a semi-blitz strategy. The best approach for success in an early campaign was to assemble all your troops in Franconia and Bohemia and hit the Polish asap. You should be able to take them out with your starting troops within 2 turns, while they - most probably - didn't expand yet. 1 faction out. Build towers, basic farming and forts everywhere else. Keep strong garrisons in Burgundy and Lorraine (important!), Austria and Poland. A small garrison in Denmark might also be useful.
Turtling, farming and teching is important now. Make use of any opportunities but otherwise lay low. In my game, the French were overrunning my allies the English, so I was forced to intervene on their behalf and secure Flanders, Champagne, Ile de France. That usually cripples them. Otherwise I only counterattacked. I took over northern Italy when they died out, so I took over. In 1240ish, Hungary finally attacked, and I'm currently finishing them off.
The Turks have recently been killed by the Byz, but everybody else is doing fine so far, which is good.

Taking out Poland early gets one risk factor of your back, and leaves some rebel provinces in eastern europe that you can take your time with. Plus their provinces aren't exactly poor.