This game was available on both cassette (!) and floppy disk, but my memory is that I played the game on disk. It was so long ago now, I'm not entirely certain about that detail, however.
I've written here before how I'd like to see a consistent set of aesthetic criteria applied to video games because I do consider them an art form. When I think about it, Zombies is likely the game that began me thinking along those terms, even as a teenager. The game was beautiful to behold, and completely immersing. For instance, I remember (I hope correctly...) that when the zombies touched you during the game, your life energy would bleed away quickly, but also that the game screen would pop and crackle, like you had been struck by electricity. If you ask me, I can still "feel" that shock, though of course, no such physical shock was actually delivered.
I attempted to download a version of Zombies when I was preparing this post, but my Sony Vaio wouldn't accept the game format, dammit.
The graphics may look primitive today, but the game play was absolutely incredible. And the sound effects are pretty much as I remember, though at least one of these videos showcases the game for Commodore 64, rather than the Atari 800.
Makes me miss my Atari 800...