i agree with that guy
an idea would be to limit archers and suchlike as the dude before me explained, but id say add more power-ups, more expensively onto it, and to make the later levels difficulty curve shoot up dramatically, to keep it hard all the way through, talking thousands of everything rushing forward for you. of course its easy to criticise, and for the first few levels its reasonably hard, but when u get to around level 22 your jus wondering when its going to end.
amazing fun tho until that point.