deciding on what provinces to take, keep and build on

[caravel]

When facing the horde, fortifications are the key. You can pull out and go under siege allowing them to divide and spread out, besieging a few of your provinces. Those should be defended by 1 or 2 units of high valour chivalric knights (dismounted), and nothing more. This will give them more time to hold out in a siege. If you can beat off their siege breaking attempts (which I find that I can, 90% of the time) the following year you should get a loyalist revolt (if you've got your spies in there and didn't build border forts! ) and be able to back this up with a counter attack to drive them out. This is better than pulling out and razing.