Okay, so it's not horribly original. But it's a reasonable second game after the trial bomber game. This shows a mock up.
I decided against shields, because it would reduce the invaders space (err...) - there probably won't be any saucer across the top either.
There's a limit to what you can draw in a 16x16 grid !
I've also updated the bomber.asm and tms1100 assembler, so it can use include files - I did it just by concatenating the files together before but that meant the line numbers on the error messages were wrong.