Fred Nachbaur’s Christmas present to his customers in 1987.
This one’s claim to fame is that it used the SHREB package, but most of the program runs in machine-code, demonstrating that the SHREB package is a useful library of graphics routines as well as a BASIC extension. This program has a bug that’s so charming I left it in. If you sideswipe an invader, it kills it but it doesn’t vaporise… it just sort of hangs there in space. The same bug also causes occasional rocket shrapnel to stay in place. Warning; this game is addictive, and hard to beat.