Thanks. The enemies are actually sprites 1-4 multiplexed, but there is a somewhat abstract data structure involved. I am sure you will figure it out :) If you are curious about how to do it, I have written a little about it here:
https://backtobasic.substack.com/p/back-to-1-mhz
You can also have a look at the source code here:
https://github.com/sveinjo/Space-Invaders-64