9-bit Alien Gunship :: Pixel Art Detail

"But why are only three of the jets working?
Is it a way of turning the ship?"

Those aren't jets.  They're an array of massive laser cannons.  They basically fire a symphony of laser blasts at the player ship(s) while the smaller purple turrets are hounding the players with bullets.  The tiny red lights on each of the cannons will light up briefly right before it fires, so it gives the player some kind of minimal warning beforehand.

"The stars are a bit repetitive and could use some work."

Yea, the stars are actually sprites (and the boss is really made of background tiles) so they must be used sparingly, otherwise you get flicker if you have too many sprites on any one horizontal line.  See, On the TG16/PCE, there is a trick you can do that makes sprites appear to move behind the background tiles, which is what I'll be employing here.  There's no way you could make a sprite as large as the lv2 boss without gettting massive amounts of flicker and/or slowdown.  Especially if you consider all the other things that will be happening on the screen (player ships firing, bullets, missiles, options, etc.)

Good job on this.  I like it!

But why are only three of the jets working?
Is it a way of turning the ship?