Don't repair your castle on the early levels. It may be tempting, but in the end, the only way to progress is to upgrade your skills. If you do this, you should be able to complete most of the game without receiving any damage at all. Once you've upgraded everything, it's very easy and you'll easily have enough to spend on repairs.
That worked pretty well. I'd figured out the archers and how to kill the shield-bearers... and the advice about castle repair was right on.
I had a little trouble on the game myself, as I'm a bit OCD when it comes to defence games - I love to have everything at 100% health. On this game though it's really vital you upgrade everything as quickly as possible. If you don't, you will be overwhelmed by about level 6/7.
i beat the ghame. i got to wave 52. (extended.) after you get the hang of it, you can play until you get bored. i just can't get the 3rd trophy. it says you have to blow up 10 people. i don't understand what your supposed to do. how do you BLOW someone up?