I really like the game and think it has great potential, but there are a few tweeks I would like to see.
1) Balloons that cross paths should attack eachother to add an element of defence for the main castle and strategy for ballon usage.
2) The windmills seem ineffectual with the increase of population production. If you speed it up then it would be a useful tool instead of a drain of resources to capture.
3) Just for fun, I would make the floating trees pop balloons to add an element of timing to the attacks.
4) Make it possible to reinforce captured towers to prevent constantly traiding off between AI and yourself.
5) Allow huts to give the ability to send additional balloons instead of the max 3. Having a stockpile of population doesn't do me much good if I can only send out 3 at a time.
As it stands, I was able to defeat every enemy by attacking their castle directly and not wasting resources on other towers. I really think this game has potential and would love to see some more strategy elements in it. Nice graphics, controls work well and the music is well chosen for the environment. Keep up the great work.