Besieged never took over tho, they would just leave. And it was always at a specific time, not spontanous.
That's completely wrong. Besieged was not at set times. and they don't "take over" because that's not their objective. Their objective is the Candy, and that's the only reason they're attacking. However, they also take prisoners, and the city does change as a result. the impact lasts until something is done about it by the players. It's really no different except in level of sophistication- which ffxi did a lot with considering the limitations it was under.