In MoM, you needed at least one unit in order to raze a city. And after razing, some goshts rose and attacked (or that was in AOW?).
BTW, monsters don't need to wait at all, so I find this mechanics totally unfair and not justified. I tamed a hoarder spider and sent it to a city, and still took 5 turns to raze...an untamed hoarder, 1 turn, and tamed not? Come on...
@GFireflyE: Don't underestimate the skills of destruction. Usually, time requiered for destructing something is inversely proportional to the time requiered in constructing it....
After all, if a turn is a season, 3 months....that's more than enough to turn a level 5 (or even bigger) city into dust.