Very Enjoyable Game
I absolutely LOVE strategic games, particularly defense games. And this is one of the best I've played in a long time.
If you want a quick fix of a "destroy all in sight" type game, then this probably isn't for you, because you have to be patient and learn what works best in different situations.
For a few times, I'd gotten as far as the first golden balloon level (on easy setting), which is about five rounds in. In that round, there's a Red Zeppelin (you're introduced to Zeppelins in round 2) that has been painted up to look like some sort of fish, or something.
Before you get to that round, ensure that you have some sort of armour and that you have repair enabled.... Because if you don't have either of those enabled, then it drops something on your main artillery that will destroy it in one blow.
My strategies so far (and I haven't gotten too far into the game, even on easy level) are to:
1. Increase shell damage from the get-go (most important for me so far).
2. Increase shell speed.
3. Increase accuracy.
4. Increase firing power.
5. Increase ammo capacity.
6. Increase reload speed.
ALL of these can be found to the left of the upgrades menu.
Then, I went to:
7. Upgrade building armour.
8. Upgrade auto repair buildings.
Both of those are in the middle of the upgrade options.
9. Upgrade winged bombs. Quite a bit.
10. Upgrade airstrike.
Both of those are in the far right column of the upgrades.
Fantastic game... Wish I had longer to play it!