I guess by chance then that's how I built. I have no idea what people are talking about with spinning zombies, and I played at least 500 hours of A16. Never seen it.
I played several A16 maps well into high gamestage. 6000 was max back then and we hit 4000+ a few times (day ~200 as I recall). We built massive Steel fortresses with several layers of traps and lost a few of them even then to simply massive hordes that were incredibly fun to fight. Mindless but massive. Lasted well into the next morning. We've had our base dug under, had walls breached, had them come over the top because the body piles outside got so high. Everything. Absolutely amazing. A17/18 is not a patch on those horde nights, because the hyper intelligent AI would make it impossible, and has reduced base building to something of a formula/puzzle, which I greatly dislike.