in Populous II - yep, on my trusty Amiga 500. It was a level where the enemy had access to 'plague' and nearly killed off all my guys... it's still fresh in my mind (which is a bit sad really!) as it was such an epic struggle to isolate my few healthy citizens, build them up to a respectable population again, and try and stave off the enemy building a bridge to my healthy people so the infected ones could get through. Pretty clever AI really, come to think of it.