How is this persons strategey?

[Gilrandir]

I remember that it can be tricky - going after Sweden and Norway as they are fairly poor provinces early on seemed to actually drain me so that I couldn't build much and large garrisons had to be kept to prevent revolts.

I have already said what has to be done to keep a province loyal - watchtowers and border forts, a governor with dread and spies. The latter is the cheapest and most efficient. As for economic potential of the provinces in question: it seems to be scanty, but you have to built a port, a trader (and its prospective upgrades) and ships, of course. Then florins would flow into your coffins. But it is true for any province the Danes can get hold of in the first decades of the game (like Saxony, for instance) - no rich province for them within an easy reach. Plus Sweden has iron - so if you start training huscarls there with armor and weapon upgrades your armies would be unstoppable.