I originally thought that this was going to be my easiest write-up so far (well, except for Peggle Nights...) - I was going to say "King & Balloon is just a shit version of Galaxian". But then I played it a few more times, and decided that was a bit harsh, and probably wrong.
There's not too much I can say about it... you're in a castle, and the king is running around underneath you. Floating above the castle are rows of balloons. I can only presume they're robot balloons, remotely controlled by some unseen enemy bent on kidnapping your king, because they don't just float there, they swoop down and you have to shoot them.
Yep. It's insane.
If they get past you, they'll sit at the bottom of the screen. The panicking king blunders about without much thought, and at some point will wander into a balloon, which will then fly off with the king. If you can shoot the balloon, the king will float back to safety. If not, you lose a life, and another king is crowned for you to save!
And that's about it. So, King & Balloon - not a shit version of Galaxian, just a weird one, that's not as good as Galaxian. Worth a play, but nothing to get too excited about.