Excellent idea...
...but the game gets dull after you realise it's always the same thing. My advise to make the game interesting is that you should make it so a different strategy is necessary to beat each level. Of course this implies that the castle must have more defending resources. As people have pointed out, now all you need to beat each level is a load of infantries (at least up to level 6, when I stopped playing).
Also, say what are the upgrades for exactly. And what is the use of buying buildings?!