It keeps overheating my laptop... it's fun and all, but just when I start doing well, -BRRZAP- AW, CRUD! I guess that's what I get for getting an eMachine, but still. Anyway, it's a very fun game.
On the original castle, try putting your treasure behing one of the long walls on the left or right. Like this:
I haven't gotten a chance to use the other castles, but with this one, half the time Skarl will completely miss your castle, leaving it 100% intact for the next round. Don't forget to practice your aim!