Quick tips

Your FlaK fires with some inaccuracy, use this to your advangtage. Find dense points and areas to cover with your fire for maximum damage.
Each air raid request costs more and more after each call (there is a ceiling of 15.000), use them only if you really need to.
The longer a balloon survives of the field, the less score you gain by destroying it.
If a balloon with not much damage comes too close, don't waste your fire on it, look for other baloons which you can still prevent from dropping bombs.
Every building you place becomes stronger over time (oil rigs produce more score, cannons fire faster and/or stronger), so keep them alive as long as you can!
After leveling up a few times, it's worth replaying easier levels to earn a higher score. The score difference adds to your total score, which can level you up again.
Try to earn gold medals, they raise your final stage score a lot.
You can see military rank markers above the scores for each levels. The highest rank marker is 5 gold starts.
There are hidden levels which appear only if you complete some specific levels on the map with the highest rank available.
Balloons have different behavior and parameters according to their colors/textures. Many of them are equipped with multiple bombs.
If a balloon survives floating through your area, it comes in for an other round, reloaded with bombs, having some damage healed and an increased speed.
Balloons are getting tougher towards the end of the stage, so try to keep your defenses alive.

This is an old web browser game.