I sort of agree with the guide above... the numbers are mainly where I differ...
I think that for the first round you get the number of snowmen surrounded +1 catapults. Afterwards you get #snowmen catapults surrounded each round... That's my experience.
I've gotten 3k points

And I must rescind my previous statement. Rotating is good, at least on the first half dozen or so levels (i really lost count of how many levels I got through).
My biggest tip: At the end of every round of attack, do not click continue right away. First you should take a moment to look at the left side of your defense (right isn't visible) and note any holes.
In the later levels it gets harder to fix your defense because you get the larger pieces more often (when what you want are the 1x2 pieces or just 1x1s). The cross takes up a lot of room, and by the end you've covered most of the screen with blocks, but with lots of little holes....
It's a fun game... 500 points = 1000 NP, very fast IMO.