I'm terrible at these kind of games, but I can see the amazing amount of potential this has. It really reminds me of Bowmaster Prelude which I loved that when I was like 7 or 8. You could add a lotta stuff here, like more towers, minions for your own castle, different maps and maybe a difficulty scale.
I'm rating well because I think it's a solid first idea for a game and if you develop this even further I'm sure it'll be a hit.