Snow Wars II is pretty easy once you have the right strategy.
First, you need to only play when you have the right snowman setup to defend. All your snowmen should be at least thee snow blocks from the edge of the field you can build in. Why three? Because that is the max length of the pieces you get to surround and re-build your fort. If you have less room than that, you end up not being able to use larger pieces to surround your snowmen in later rounds.
So, restart the game until your snowmen are in a good place. Then build a fort around them. I usually just click like mad on my mouse in a rough circle around the snowmen and fill in spots I miss.